diff --git a/labs/04/arithmetic.ex b/labs/04/arithmetic.ex
new file mode 100644
index 0000000..63ed218
--- /dev/null
+++ b/labs/04/arithmetic.ex
@@ -0,0 +1,90 @@
+defmodule Arithmetic.Worker do
+  use GenServer
+
+  def start_link() do
+    GenServer.start_link(__MODULE__, nil)
+  end
+
+  ### Server Impl ###
+  @impl true
+  def init(_arg) do
+    {:ok, nil}
+  end
+
+  @impl true
+  def handle_cast({:square, x, from}, state) do
+    # The server dispatches work asynchronously, so workers handle casts and
+    # reply directly to the client that made the original GenServer.call/2.
+    GenServer.reply(from, {self(), x * x})
+    {:noreply, state}
+  end
+
+  @impl true
+  def handle_cast({:sqrt, x, from}, state) do
+    Process.sleep(2000)
+    GenServer.reply(from, {self(), if(x < 0, do: :error, else: :math.sqrt(x))})
+    {:noreply, state}
+  end
+end
+
+defmodule Arithmetic.Server do
+  use GenServer
+
+  ### Client API ###
+  def start(n) do
+    GenServer.start(__MODULE__, n, name: __MODULE__)
+  end
+
+  def square(x) do
+    GenServer.call(__MODULE__, {:square, x})
+  end
+
+  def sqrt(x) do
+    GenServer.call(__MODULE__, {:sqrt, x})
+  end
+
+  ### Server Impl ###
+  @impl true
+  def init(n) do
+    Process.flag(:trap_exit, true)
+
+    workers =
+      Enum.map(1..n, fn _ ->
+        {:ok, pid} = Arithmetic.Worker.start_link()
+        IO.inspect(pid)
+        pid
+      end)
+
+    {:ok, {workers, 0}}
+  end
+
+  @impl true
+  def handle_call({:square, x}, from, {workers, index}) do
+    worker = Enum.at(workers, index)
+
+    GenServer.cast(worker, {:square, x, from})
+
+    next = rem(index + 1, length(workers))
+    # Do not reply here; the selected worker replies to `from` after the cast.
+    {:noreply, {workers, next}}
+  end
+
+  @impl true
+  def handle_call({:sqrt, x}, from, {workers, index}) do
+    worker = Enum.at(workers, index)
+
+    GenServer.cast(worker, {:sqrt, x, from})
+
+    next = rem(index + 1, length(workers))
+    {:noreply, {workers, next}}
+  end
+
+  @impl true
+  def handle_info({:EXIT, dead, reason}, {workers, index}) do
+    {:ok, new} = Arithmetic.Worker.start_link()
+    IO.puts("Worker #{inspect(dead)} died (#{inspect(reason)}), replacement: #{inspect(new)}")
+
+    workers = Enum.map(workers, fn pid -> if pid == dead, do: new, else: pid end)
+    {:noreply, {workers, index}}
+  end
+end
diff --git a/labs/04/card/.formatter.exs b/labs/04/card/.formatter.exs
new file mode 100644
index 0000000..d2cda26
--- /dev/null
+++ b/labs/04/card/.formatter.exs
@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]
diff --git a/labs/04/card/.gitignore b/labs/04/card/.gitignore
new file mode 100644
index 0000000..acf9971
--- /dev/null
+++ b/labs/04/card/.gitignore
@@ -0,0 +1,28 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+card-*.tar
+
+# Temporary files, for example, from tests.
+/tmp/
+
+cards.db
diff --git a/labs/04/card/README.md b/labs/04/card/README.md
new file mode 100644
index 0000000..4f4836e
--- /dev/null
+++ b/labs/04/card/README.md
@@ -0,0 +1,21 @@
+# Card
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `card` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:card, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at <https://hexdocs.pm/card>.
+
diff --git a/labs/04/card/lib/card/application.ex b/labs/04/card/lib/card/application.ex
new file mode 100644
index 0000000..c941145
--- /dev/null
+++ b/labs/04/card/lib/card/application.ex
@@ -0,0 +1,16 @@
+defmodule Card.Application do
+  @moduledoc false
+
+  use Application
+
+  @impl true
+  def start(_type, _args) do
+    children = [
+      {Card.Store, "cards.db"},
+      Card.Worker
+    ]
+
+    opts = [strategy: :one_for_one, name: Card.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+end
diff --git a/labs/04/card/lib/card/store.ex b/labs/04/card/lib/card/store.ex
new file mode 100644
index 0000000..86a4879
--- /dev/null
+++ b/labs/04/card/lib/card/store.ex
@@ -0,0 +1,39 @@
+defmodule Card.Store do
+  use GenServer
+
+  # client
+  def start_link(file) do
+    GenServer.start_link(__MODULE__, file, name: __MODULE__)
+  end
+
+  def put(value) do
+    GenServer.call(__MODULE__, {:put, value})
+  end
+
+  def get() do
+    GenServer.call(__MODULE__, :get)
+  end
+
+  # server
+  @impl true
+  def init(file) do
+    {:ok, file}
+  end
+
+  @impl true
+  def handle_call(:get, _from, file) do
+    reply =
+      case File.read(file) do
+        {:ok, content} -> :erlang.binary_to_term(content)
+        _ -> nil
+      end
+
+    {:reply, reply, file}
+  end
+
+  @impl true
+  def handle_call({:put, value}, _from, file) do
+    reply = File.write(file, :erlang.term_to_binary(value))
+    {:reply, reply, file}
+  end
+end
diff --git a/labs/04/card/lib/card/worker.ex b/labs/04/card/lib/card/worker.ex
new file mode 100644
index 0000000..f935462
--- /dev/null
+++ b/labs/04/card/lib/card/worker.ex
@@ -0,0 +1,79 @@
+defmodule Card.Worker do
+  use GenServer
+
+  ### Client API ###
+  def start_link(_arg \\ nil) do
+    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
+  end
+
+  def new() do
+    GenServer.cast(__MODULE__, :new)
+  end
+
+  def shuffle() do
+    GenServer.cast(__MODULE__, :shuffle)
+  end
+
+  def count() do
+    GenServer.call(__MODULE__, :count)
+  end
+
+  def deal(n \\ 1) do
+    GenServer.call(__MODULE__, {:deal, n})
+  end
+
+  ### Server Impl ###
+  defp fresh() do
+    values = Enum.map(2..10, &Integer.to_string/1) ++ ["J", "Q", "K", "A"]
+    suits = ["♣", "♦", "♥", "♠"]
+
+    for v <- values, f <- suits, do: v <> f
+  end
+
+  defp save(state) do
+    Card.Store.put(state)
+    state
+  end
+
+  @impl true
+  def init(_arg) do
+    IO.puts("Card.Worker started/re-started")
+    {:ok, Card.Store.get() || save(fresh())}
+  end
+
+  @impl true
+  def handle_cast(:new, _state) do
+    # {:noreply, new_state}
+    {:noreply, save(fresh())}
+  end
+
+  @impl true
+  def handle_cast(:shuffle, state) do
+    # {:noreply, new_state}
+    {:noreply, save(Enum.shuffle(state))}
+  end
+
+  @impl true
+  def handle_call(:count, _from, state) do
+    # {:reply, actual_reply, new_state}
+    {:reply, length(state), state}
+  end
+
+  @impl true
+  def handle_call({:deal, n}, _from, state) when is_integer(n) do
+    case n do
+      _ when n < 0 ->
+        # {:reply, actual_reply, new_state}
+        {:reply, {:error, "Cannot deal negative number of cards"}, state}
+
+      _ when n > length(state) ->
+        # {:reply, actual_reply, new_state}
+        {:reply, {:error, "Insufficient cards in deck to deal"}, state}
+
+      _ ->
+        {d, r} = Enum.split(state, n)
+        # {:reply, actual_reply, new_state}
+        {:reply, {:ok, d}, save(r)}
+    end
+  end
+end
diff --git a/labs/04/card/mix.exs b/labs/04/card/mix.exs
new file mode 100644
index 0000000..3626383
--- /dev/null
+++ b/labs/04/card/mix.exs
@@ -0,0 +1,29 @@
+defmodule Card.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :card,
+      version: "0.1.0",
+      elixir: "~> 1.17",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {Card.Application, []}
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end
diff --git a/labs/04/card/test/test_helper.exs b/labs/04/card/test/test_helper.exs
new file mode 100644
index 0000000..869559e
--- /dev/null
+++ b/labs/04/card/test/test_helper.exs
@@ -0,0 +1 @@
+ExUnit.start()
diff --git a/labs/04/lab4/arithmetic.ex b/labs/04/lab4/arithmetic.ex
new file mode 100644
index 0000000..63ed218
--- /dev/null
+++ b/labs/04/lab4/arithmetic.ex
@@ -0,0 +1,90 @@
+defmodule Arithmetic.Worker do
+  use GenServer
+
+  def start_link() do
+    GenServer.start_link(__MODULE__, nil)
+  end
+
+  ### Server Impl ###
+  @impl true
+  def init(_arg) do
+    {:ok, nil}
+  end
+
+  @impl true
+  def handle_cast({:square, x, from}, state) do
+    # The server dispatches work asynchronously, so workers handle casts and
+    # reply directly to the client that made the original GenServer.call/2.
+    GenServer.reply(from, {self(), x * x})
+    {:noreply, state}
+  end
+
+  @impl true
+  def handle_cast({:sqrt, x, from}, state) do
+    Process.sleep(2000)
+    GenServer.reply(from, {self(), if(x < 0, do: :error, else: :math.sqrt(x))})
+    {:noreply, state}
+  end
+end
+
+defmodule Arithmetic.Server do
+  use GenServer
+
+  ### Client API ###
+  def start(n) do
+    GenServer.start(__MODULE__, n, name: __MODULE__)
+  end
+
+  def square(x) do
+    GenServer.call(__MODULE__, {:square, x})
+  end
+
+  def sqrt(x) do
+    GenServer.call(__MODULE__, {:sqrt, x})
+  end
+
+  ### Server Impl ###
+  @impl true
+  def init(n) do
+    Process.flag(:trap_exit, true)
+
+    workers =
+      Enum.map(1..n, fn _ ->
+        {:ok, pid} = Arithmetic.Worker.start_link()
+        IO.inspect(pid)
+        pid
+      end)
+
+    {:ok, {workers, 0}}
+  end
+
+  @impl true
+  def handle_call({:square, x}, from, {workers, index}) do
+    worker = Enum.at(workers, index)
+
+    GenServer.cast(worker, {:square, x, from})
+
+    next = rem(index + 1, length(workers))
+    # Do not reply here; the selected worker replies to `from` after the cast.
+    {:noreply, {workers, next}}
+  end
+
+  @impl true
+  def handle_call({:sqrt, x}, from, {workers, index}) do
+    worker = Enum.at(workers, index)
+
+    GenServer.cast(worker, {:sqrt, x, from})
+
+    next = rem(index + 1, length(workers))
+    {:noreply, {workers, next}}
+  end
+
+  @impl true
+  def handle_info({:EXIT, dead, reason}, {workers, index}) do
+    {:ok, new} = Arithmetic.Worker.start_link()
+    IO.puts("Worker #{inspect(dead)} died (#{inspect(reason)}), replacement: #{inspect(new)}")
+
+    workers = Enum.map(workers, fn pid -> if pid == dead, do: new, else: pid end)
+    {:noreply, {workers, index}}
+  end
+end
diff --git a/labs/04/lab4/card/.formatter.exs b/labs/04/lab4/card/.formatter.exs
new file mode 100644
index 0000000..d2cda26
--- /dev/null
+++ b/labs/04/lab4/card/.formatter.exs
@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]
diff --git a/labs/04/lab4/card/.gitignore b/labs/04/lab4/card/.gitignore
new file mode 100644
index 0000000..acf9971
--- /dev/null
+++ b/labs/04/lab4/card/.gitignore
@@ -0,0 +1,28 @@
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where third-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
+erl_crash.dump
+
+# Also ignore archive artifacts (built via "mix archive.build").
+*.ez
+
+# Ignore package tarball (built via "mix hex.build").
+card-*.tar
+
+# Temporary files, for example, from tests.
+/tmp/
+
+cards.db
diff --git a/labs/04/lab4/card/README.md b/labs/04/lab4/card/README.md
new file mode 100644
index 0000000..4f4836e
--- /dev/null
+++ b/labs/04/lab4/card/README.md
@@ -0,0 +1,21 @@
+# Card
+
+**TODO: Add description**
+
+## Installation
+
+If [available in Hex](https://hex.pm/docs/publish), the package can be installed
+by adding `card` to your list of dependencies in `mix.exs`:
+
+```elixir
+def deps do
+  [
+    {:card, "~> 0.1.0"}
+  ]
+end
+```
+
+Documentation can be generated with [ExDoc](https://github.com/elixir-lang/ex_doc)
+and published on [HexDocs](https://hexdocs.pm). Once published, the docs can
+be found at <https://hexdocs.pm/card>.
+
diff --git a/labs/04/lab4/card/lib/card/application.ex b/labs/04/lab4/card/lib/card/application.ex
new file mode 100644
index 0000000..c941145
--- /dev/null
+++ b/labs/04/lab4/card/lib/card/application.ex
@@ -0,0 +1,16 @@
+defmodule Card.Application do
+  @moduledoc false
+
+  use Application
+
+  @impl true
+  def start(_type, _args) do
+    children = [
+      {Card.Store, "cards.db"},
+      Card.Worker
+    ]
+
+    opts = [strategy: :one_for_one, name: Card.Supervisor]
+    Supervisor.start_link(children, opts)
+  end
+end
diff --git a/labs/04/lab4/card/lib/card/store.ex b/labs/04/lab4/card/lib/card/store.ex
new file mode 100644
index 0000000..86a4879
--- /dev/null
+++ b/labs/04/lab4/card/lib/card/store.ex
@@ -0,0 +1,39 @@
+defmodule Card.Store do
+  use GenServer
+
+  # client
+  def start_link(file) do
+    GenServer.start_link(__MODULE__, file, name: __MODULE__)
+  end
+
+  def put(value) do
+    GenServer.call(__MODULE__, {:put, value})
+  end
+
+  def get() do
+    GenServer.call(__MODULE__, :get)
+  end
+
+  # server
+  @impl true
+  def init(file) do
+    {:ok, file}
+  end
+
+  @impl true
+  def handle_call(:get, _from, file) do
+    reply =
+      case File.read(file) do
+        {:ok, content} -> :erlang.binary_to_term(content)
+        _ -> nil
+      end
+
+    {:reply, reply, file}
+  end
+
+  @impl true
+  def handle_call({:put, value}, _from, file) do
+    reply = File.write(file, :erlang.term_to_binary(value))
+    {:reply, reply, file}
+  end
+end
diff --git a/labs/04/lab4/card/lib/card/worker.ex b/labs/04/lab4/card/lib/card/worker.ex
new file mode 100644
index 0000000..f935462
--- /dev/null
+++ b/labs/04/lab4/card/lib/card/worker.ex
@@ -0,0 +1,79 @@
+defmodule Card.Worker do
+  use GenServer
+
+  ### Client API ###
+  def start_link(_arg \\ nil) do
+    GenServer.start_link(__MODULE__, nil, name: __MODULE__)
+  end
+
+  def new() do
+    GenServer.cast(__MODULE__, :new)
+  end
+
+  def shuffle() do
+    GenServer.cast(__MODULE__, :shuffle)
+  end
+
+  def count() do
+    GenServer.call(__MODULE__, :count)
+  end
+
+  def deal(n \\ 1) do
+    GenServer.call(__MODULE__, {:deal, n})
+  end
+
+  ### Server Impl ###
+  defp fresh() do
+    values = Enum.map(2..10, &Integer.to_string/1) ++ ["J", "Q", "K", "A"]
+    suits = ["♣", "♦", "♥", "♠"]
+
+    for v <- values, f <- suits, do: v <> f
+  end
+
+  defp save(state) do
+    Card.Store.put(state)
+    state
+  end
+
+  @impl true
+  def init(_arg) do
+    IO.puts("Card.Worker started/re-started")
+    {:ok, Card.Store.get() || save(fresh())}
+  end
+
+  @impl true
+  def handle_cast(:new, _state) do
+    # {:noreply, new_state}
+    {:noreply, save(fresh())}
+  end
+
+  @impl true
+  def handle_cast(:shuffle, state) do
+    # {:noreply, new_state}
+    {:noreply, save(Enum.shuffle(state))}
+  end
+
+  @impl true
+  def handle_call(:count, _from, state) do
+    # {:reply, actual_reply, new_state}
+    {:reply, length(state), state}
+  end
+
+  @impl true
+  def handle_call({:deal, n}, _from, state) when is_integer(n) do
+    case n do
+      _ when n < 0 ->
+        # {:reply, actual_reply, new_state}
+        {:reply, {:error, "Cannot deal negative number of cards"}, state}
+
+      _ when n > length(state) ->
+        # {:reply, actual_reply, new_state}
+        {:reply, {:error, "Insufficient cards in deck to deal"}, state}
+
+      _ ->
+        {d, r} = Enum.split(state, n)
+        # {:reply, actual_reply, new_state}
+        {:reply, {:ok, d}, save(r)}
+    end
+  end
+end
diff --git a/labs/04/lab4/card/mix.exs b/labs/04/lab4/card/mix.exs
new file mode 100644
index 0000000..3626383
--- /dev/null
+++ b/labs/04/lab4/card/mix.exs
@@ -0,0 +1,29 @@
+defmodule Card.MixProject do
+  use Mix.Project
+
+  def project do
+    [
+      app: :card,
+      version: "0.1.0",
+      elixir: "~> 1.17",
+      start_permanent: Mix.env() == :prod,
+      deps: deps()
+    ]
+  end
+
+  # Run "mix help compile.app" to learn about applications.
+  def application do
+    [
+      extra_applications: [:logger],
+      mod: {Card.Application, []}
+    ]
+  end
+
+  # Run "mix help deps" to learn about dependencies.
+  defp deps do
+    [
+      # {:dep_from_hexpm, "~> 0.3.0"},
+      # {:dep_from_git, git: "https://github.com/elixir-lang/my_dep.git", tag: "0.1.0"}
+    ]
+  end
+end
diff --git a/labs/04/lab4/card/test/test_helper.exs b/labs/04/lab4/card/test/test_helper.exs
new file mode 100644
index 0000000..869559e
--- /dev/null
+++ b/labs/04/lab4/card/test/test_helper.exs
@@ -0,0 +1 @@
+ExUnit.start()