diff --git a/.github/workflows/Linux.yml b/.github/workflows/Linux.yml
index bc8de79..e9d9e60 100644
--- a/.github/workflows/Linux.yml
+++ b/.github/workflows/Linux.yml
@@ -16,6 +16,7 @@ jobs:
           # Decision on version matrix informed by https://discuss.ocaml.org/t/which-ocaml-compiler-versions-should-we-run-against-in-ci/7933/2
           # But has gradually inched up due to signs of bitrot on earlier versions
           # such as https://github.com/thierry-martinez/stdcompat/issues/26
+          - 4.x
           - 5.x
     runs-on: ${{ matrix.os }}
     steps:
diff --git a/.github/workflows/MacOS.yml b/.github/workflows/MacOS.yml
index 3af0ea3..26982a1 100644
--- a/.github/workflows/MacOS.yml
+++ b/.github/workflows/MacOS.yml
@@ -16,6 +16,7 @@ jobs:
           # Decision on version matrix informed by https://discuss.ocaml.org/t/which-ocaml-compiler-versions-should-we-run-against-in-ci/7933/2
           # But has gradually inched up due to signs of bitrot on earlier versions
           # such as https://github.com/thierry-martinez/stdcompat/issues/26
+          - 4.x
           - 5.x
     runs-on: ${{ matrix.os }}
     steps:
diff --git a/.github/workflows/Windows.yml b/.github/workflows/Windows.yml
index a04c540..4dd15e6 100644
--- a/.github/workflows/Windows.yml
+++ b/.github/workflows/Windows.yml
@@ -16,7 +16,7 @@ jobs:
           # Decision on version matrix informed by https://discuss.ocaml.org/t/which-ocaml-compiler-versions-should-we-run-against-in-ci/7933/2
           # But has gradually inched up due to signs of bitrot on earlier versions
           # such as https://github.com/thierry-martinez/stdcompat/issues/26
-          - 5.x
+          - 4.x
     runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout code
diff --git a/README.md b/README.md
index 5d3dfe3..bf5bf98 100644
--- a/README.md
+++ b/README.md
@@ -21,47 +21,35 @@ __An OCaml minimalist testing framework with zero dependencies.__
 ## Usage
 [./test/test_omtl.ml](./test/test_omtl.ml)
 ```ocaml
-open Omtl
-
 (* A module with functions to test *)
-module My_String = struct
-  let equal = String.equal
-
-  let capitalize = String.capitalize_ascii
+module Suit = struct
+  let pass () = ()
 
-  let str_concat = String.concat ""
+  let failed () = "Failed !!!"
 end
 
 (* The tests *)
-let test_equal () =
-  if My_String.equal "hello!" "hello!" then
-    ()
-  else
-    fail {| My_String.equal "hello!" = "hello!" |}
-
-let test_capitalize () =
-  if String.equal "HELLO!" (My_String.capitalize "hELLO!") then
-    ()
-  else
-    fail {| My_String.capitalize "hELLO!" = "HELLO!!" |}
+let test_pass () = Suit.pass () |> ok
 
-let test_str_concat () =
-  if String.equal "foobar" (My_String.str_concat ["foo"; "bar"]) then
-    ()
-  else
-    fail {| My_String.str_concat ["foo"; "bar"] = "foobar" |}
+let test_failed () = Suit.failed () |> err
 
 let test_failure () = failwith "Take it easy, this is just an example of a failed test"
+
 let test_exception () = raise Not_found
-let test_function_running_time () = for _ = 0 to 100000 do () done
+
+let test_function_running_time () =
+  for _ = 0 to 100000 do
+    ()
+  done;
+  Ok ()
+
 
 (* Run it *)
 let _ =
   "My_String"
   +:> [
-        "equal" >== test_equal;
-        "capitalize" >== test_capitalize;
-        "str_concat" >== test_str_concat;
+        "pass" >== test_pass;
+        "failed" >== test_failed;
         "Examples of test failures" >== test_failure;
         "Examples of undefined exception" >== test_exception;
         "Test function running time" >== test_function_running_time;
diff --git a/dune-project b/dune-project
index 19ec701..e1ad898 100644
--- a/dune-project
+++ b/dune-project
@@ -22,7 +22,7 @@
  (description "OCaml Minimalist Testing Library")
  (depends
   (ocaml
-   (>= 5.0.0))
+   (>= 4.13.1))
   dune)
  (tags
   (testing "test framework")))
diff --git a/lib/color.ml b/lib/color.ml
index 3194bde..1e22b01 100644
--- a/lib/color.ml
+++ b/lib/color.ml
@@ -28,7 +28,7 @@
  *)
 
 type color =
-  | Ok
+  | Pass
   | Fail
   | Fail_info
   | Time
@@ -41,7 +41,7 @@ type color =
 
 let color_map : (color, string) Hashtbl.t =
   [
-    Ok, "\027[32m";
+    Pass, "\027[32m";
     Fail, "\027[5;31m";
     Fail_info, "\027[31m";
     Info_title, "\027[4;36m";
diff --git a/lib/info.ml b/lib/info.ml
index 23729e8..998eb62 100644
--- a/lib/info.ml
+++ b/lib/info.ml
@@ -48,15 +48,18 @@ functor
   struct
     let get () : string =
       M.get_info () |> String.split_on_char '\n' |> M.filter |> M.decorate |> String.concat "\n"
+    [@@inline always]
   end
 
 module Get_Info = struct
   module Backtrace = struct
     let get_info () = Printexc.get_raw_backtrace () |> Printexc.raw_backtrace_to_string
+    [@@inline always]
   end
 
   module CallStack = struct
     let get_info () = Printexc.get_callstack 20 |> Printexc.raw_backtrace_to_string
+    [@@inline always]
   end
 end
 
@@ -68,10 +71,11 @@ module Filter = struct
            (not (String.starts_with ~prefix:"Called from Omtl.test.time" s))
            && not (String.equal s ""))
         backtraces
+    [@@inline always]
   end
 
   module CallStack = struct
-    let filter (lst : string list) : string list = lst
+    let filter (lst : string list) : string list = lst [@@inline always]
   end
 end
 
diff --git a/lib/omtl.ml b/lib/omtl.ml
index 6e4c0b1..d99a720 100644
--- a/lib/omtl.ml
+++ b/lib/omtl.ml
@@ -44,7 +44,7 @@ let test status (f : unit -> (unit, string) Result.t) : Test_Result.t =
           Result.map (fun () -> Standalone_unix.gettimeofday () -. timer) result
         in
         match time f with
-        | Result.Ok time -> Test_Result.Ok time
+        | Result.Ok time -> Test_Result.Pass time
         | Result.Error info ->
             let backtrace = if backtrace then Backtrace.get () else String.empty
             and callstack = if callstack then CallStack.get () else String.empty in
@@ -63,12 +63,12 @@ let test_case status (test_case : test_case) : string =
   | { backtrace; callstack; force; suit = _ } -> (
       let name, f = test_case in
       match test status f with
-      | Test_Result.Ok time ->
+      | Test_Result.Pass time ->
           Format.sprintf
             "\t %s- %s...%s %s"
-            (text ~force ~color:Ok "o")
+            (text ~force ~color:Pass "o")
             name
-            (text ~force ~color:Ok "OK")
+            (text ~force ~color:Pass "OK")
             (text ~force ~color:Time (Format.sprintf "(%f ms)" (time *. 1000.)))
       | Test_Result.Fail (i, b, c) ->
           Format.sprintf
diff --git a/lib/types.ml b/lib/types.ml
index 88b2963..9004a71 100644
--- a/lib/types.ml
+++ b/lib/types.ml
@@ -40,7 +40,7 @@ and test_case = string * (unit -> (unit, string) result)
 
 module Test_Result = struct
   type t =
-    | Ok of time
+    | Pass of time
     | Fail of info * backtraces * callstack
 
   and time = float
diff --git a/lib/utils.ml b/lib/utils.ml
index 0fe3002..87a9381 100644
--- a/lib/utils.ml
+++ b/lib/utils.ml
@@ -29,10 +29,11 @@
 
 open Types
 
-(** Assist in building test sets and individual test items to improve the readability of test code *)
+(* Assist in building test sets and individual test items to improve the readability of test code *)
 let ( +:> ) (name : string) (test_case_list : test_case list) = name, test_case_list
 
 let ( >== ) (name : string) (f : unit -> (unit, string) result) : test_case = name, f
 
-(** Wrapper function to failwith *)
-let fail = failwith
+let ok v = Ok v
+
+let err v = Error v
diff --git a/omtl.opam b/omtl.opam
index cf84f66..e164f66 100644
--- a/omtl.opam
+++ b/omtl.opam
@@ -10,7 +10,7 @@ homepage: "https://github.com/muqiuhan/omtl"
 doc: "https://github.com/omtl"
 bug-reports: "https://github.com/muqiuhan/omtl/issues"
 depends: [
-  "ocaml" {>= "5.0.0"}
+  "ocaml" {>= "4.13.1"}
   "dune" {>= "3.0"}
   "odoc" {with-doc}
 ]
diff --git a/test/test_omtl.ml b/test/test_omtl.ml
index 5bcffb8..007304f 100644
--- a/test/test_omtl.ml
+++ b/test/test_omtl.ml
@@ -30,35 +30,16 @@
 open Omtl
 
 (* A module with functions to test *)
-module My_String = struct
-  let equal = String.equal
+module Suit = struct
+  let pass () = ()
 
-  let capitalize = String.capitalize_ascii
-
-  let str_concat = String.concat ""
+  let failed () = "Failed !!!"
 end
 
 (* The tests *)
-let test_equal () =
-  if My_String.equal "hello!" "hello!" then
-    Ok ()
-  else
-    Error {| My_String.equal "hello!" = "hello!" |}
-
-
-let test_capitalize () =
-  if String.equal "HELLO!" (My_String.capitalize "hELLO!") then
-    Ok ()
-  else
-    Error {| My_String.capitalize "hELLO!" = "HELLO!!" |}
-
-
-let test_str_concat () =
-  if String.equal "foobar" (My_String.str_concat ["foo"; "bar"]) then
-    Ok ()
-  else
-    Error {| My_String.str_concat ["foo"; "bar"] = "foobar" |}
+let test_pass () = Suit.pass () |> ok
 
+let test_failed () = Suit.failed () |> err
 
 let test_failure () = failwith "Take it easy, this is just an example of a failed test"
 
@@ -67,17 +48,16 @@ let test_exception () = raise Not_found
 let test_function_running_time () =
   for _ = 0 to 100000 do
     ()
-  done;
-  Ok ()
+  done
+  |> ok
 
 
 (* Run it *)
 let _ =
   "My_String"
   +:> [
-        "equal" >== test_equal;
-        "capitalize" >== test_capitalize;
-        "str_concat" >== test_str_concat;
+        "pass" >== test_pass;
+        "failed" >== test_failed;
         "Examples of test failures" >== test_failure;
         "Examples of undefined exception" >== test_exception;
         "Test function running time" >== test_function_running_time;