diff --git a/lecture04/bstree.ml b/lecture04/bstree.ml
new file mode 100644
index 0000000..6179317
--- /dev/null
+++ b/lecture04/bstree.ml
@@ -0,0 +1,65 @@
+type 'a bstree = Leaf | Node of 'a * 'a bstree * 'a bstree;;
+
+let rec bstree_size t =
+  match t with
+  | Leaf -> 0
+  | Node (_, l, r) -> 
+    1 + bstree_size l + bstree_size r;;
+
+let rec bstree_height t =
+  match t with
+  | Leaf -> 0
+  | Node (_, l, r) -> 
+    1 + max (bstree_height l) (bstree_height r);;
+
+let bstree_empty = Leaf;;
+
+let bstree_is_empty t = t = Leaf;;
+
+let rec bstree_insert ~cmp x t =
+  match t with
+  | Leaf -> Node (x, Leaf, Leaf)
+  | Node (x', l, r) when cmp x x' < 0 ->
+    Node (x', bstree_insert ~cmp x l, r)
+  | Node (x', l, r) when cmp x x' > 0 ->
+    Node (x', l, bstree_insert ~cmp x r)
+  | _ -> t;;
+
+let bstree_of_list ~cmp l =
+  List.fold_left (Fun.flip (bstree_insert ~cmp)) Leaf l;;
+
+let rec bstree_mem ~cmp x t =
+  match t with
+  | Leaf -> false
+  | Node (x', l, _) when cmp x x' < 0 ->
+    bstree_mem ~cmp x l
+  | Node (x', _, r) when cmp x x' > 0 ->
+    bstree_mem ~cmp x r
+  | _ -> true;;
+
+let rec bstree_largest t =
+  match t with
+  | Leaf -> failwith "bstree_largest: empty tree"
+  | Node (x, _, Leaf) -> x
+  | Node (_, _, r) -> bstree_largest r;;
+
+let rec bstree_smallest t =
+  match t with
+  | Leaf -> failwith "bstree_smallest: empty tree"
+  | Node (x, Leaf, _) -> x
+  | Node (_, l, _) -> bstree_smallest l;;
+
+let rec bstree_delete ~cmp x t =
+  match t with
+  | Leaf -> Leaf
+  | Node (x', l, r) when cmp x x' < 0 ->
+    Node (x', bstree_delete ~cmp x l, r)
+  | Node (x', l, r) when cmp x x' > 0 ->
+    Node (x', l, bstree_delete ~cmp x r)
+  | Node (_, Leaf, Leaf) -> Leaf (* this does not need to be here, its a special case but ill leave it for clarity *)
+  | Node (_, l, Leaf) -> l
+  | Node (_, Leaf, r) -> r
+  | Node (_, l, r) ->
+    let succ = bstree_largest l in
+    Node (succ, bstree_delete ~cmp succ l, r);;
+
diff --git a/lecture04/bstree.mli b/lecture04/bstree.mli
new file mode 100644
index 0000000..eb28917
--- /dev/null
+++ b/lecture04/bstree.mli
@@ -0,0 +1,10 @@
+type bstree_size : 'a bstree -> int
+val bstree_height : 'a bstree -> int
+val bstree_empty : 'a bstree
+val bstree_is_empty : 'a bstree -> bool
+val bstree_insert : cmp:('a -> 'a -> int) -> 'a -> 'a bstree -> 'a bstree
+val bstree_of_list : cmp:('a -> 'a -> int) -> 'a list -> 'a bstree
+val bstree_mem : cmp:('a -> 'b -> int) -> 'a -> 'b bstree -> bool
+val bstree_largest : 'a bstree -> 'a
+val bstree_smallest : 'a bstree -> 'a
+val bstree_delete : cmp:('a -> 'a -> int) -> 'a -> 'a bstree -> 'a bstree
diff --git a/lecture04/notes.ml b/lecture04/notes.ml
new file mode 100644
index 0000000..cc8d51e
--- /dev/null
+++ b/lecture04/notes.ml
@@ -0,0 +1,107 @@
+(* Some, None - Are called data/value constructors *)
+Some 1;; (* - : int option = Some 1 *)
+
+None;; (* - : 'a option = None *)
+
+(* OPTIONS are called type constructors, it takes a type *)
+(* 'a is a type variable, where 'a is some type such as int *)
+
+type 'a option = None | Some of 'a;;
+
+(*
+  You can define any type you want, this type constructor starts
+  with a lower case letter "direction" and the value constructor starts
+  with uppercase "North", "East", ... 
+
+  type constructors are not functions
+*)
+type direction = North | East | South | West;;
+
+(* There are RESULT types *)
+Ok 1;;
+Error "hell";; (* albert's example is hell not my example *)
+
+(*
+  A result is a two type variable, this is the syntax for
+  something that takes to type variables
+*)
+type ('a, 'b) result = Ok of 'a | Error of 'b;;
+
+(1, 2);; (* type is - : int * int = (1, 2) *)
+(* (int, int) : (type * type) *)
+
+(* we define an expresion that has a type of Int, and can be of type Add, Sub or Mul *)
+type expr = 
+    Int of int
+  | Add of expr * expr
+  | Sub of expr * expr
+  | Mul of expr * expr;;
+
+(* We then define an eval function, that takes in an eval,
+   and recursively evaluates e1, e2 till they are just of
+   type Int to then evaluate the addition, subtraction, etc. *)
+let rec eval = function
+  | Int n -> n
+  | Add (e1, e2) -> eval e1 + eval e2
+  | Sub (e1, e2) -> eval e1 - eval e2
+  | Mul (e1, e2) -> eval e1 * eval e2;;
+
+let e = Mul(Add (Int 1, Int 2), Int 7);;
+eval e;;
+
+(* lets apply the type variables and constructors to create a card type *)
+type suit = Club | Diamond | Heart | Spade;;
+type rank = Num of int | Jack | Queen | King | Ace;;
+type card = rank * suit;;
+
+let compare_rank r1 r2 = 
+  match r1, r2 with
+  | Num x, Num y -> Int.compare x y
+  | Num _, _ -> -1
+  | _, Num _ -> 1
+  | _, _ -> Stdlib.compare r1 r2;;
+
+let compare_suit s1 s2 = Stdlib.compare s1 s2;;
+
+let compare_card (r1, s1) (r2, s2) =
+  let c = compare_rank r1 r2 in
+  if c = 0 then compare_suit s1 s2
+  else c;;
+
+let string_of_suit = function
+  | Club -> "clubs"
+  | Diamond -> "diamonds"
+  | Heart -> "hearts"
+  | Spade -> "spades";;
+
+let string_of_rank = function
+  | Num n -> string_of_int n
+  | Jack -> "Jack"
+  | Queen -> "Queen"
+  | King -> "King"
+  | Ace -> "Ace"
+
+let string_of_card (r, s) =
+  string_of_rank r ^ " of " ^ string_of_suit s;;
+
+compare_card (King, Diamond) (Queen, Heart);;
+string_of_card (Queen, Heart);;
+
+let all_suits = [Club; Diamond; Heart; Spade];;
+let all_ranks =
+  (List.init 9 (fun x -> x + 2) |> List.map (fun x -> Num x)) @ 
+  [Jack; Queen; King; Ace];;
+
+let all_cards =
+  List.fold_right (fun rank acc -> (
+    List.fold_right (fun suit acc -> (rank, suit) :: acc) all_suits []
+  ) @ acc) all_ranks [];;
+
+type 'a linked_list = Nil | Cons of 'a * 'a linked_list;;
+
+let rec map_linked f = function
+  | Nil -> Nil
+  | Cons (x, l) -> Cons (f x, map_linked f l);;
+
+Cons (1, Cons (2, Cons (3, Nil))) |> map_linked (fun x -> x * x);;
+