diff --git a/4-testing/go.mod b/4-testing/go.mod
new file mode 100644
index 0000000..7dfef65
--- /dev/null
+++ b/4-testing/go.mod
@@ -0,0 +1,5 @@
+module example/user/testing
+
+go 1.17
+
+require github.com/google/go-cmp v0.5.6
diff --git a/4-testing/go.sum b/4-testing/go.sum
new file mode 100644
index 0000000..03e1a9c
--- /dev/null
+++ b/4-testing/go.sum
@@ -0,0 +1,4 @@
+github.com/google/go-cmp v0.5.6 h1:BKbKCqvP6I+rmFHt06ZmyQtvB8xAkWdhFyr0ZUNZcxQ=
+github.com/google/go-cmp v0.5.6/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
diff --git a/4-testing/hello.go b/4-testing/hello.go
new file mode 100644
index 0000000..c2db773
--- /dev/null
+++ b/4-testing/hello.go
@@ -0,0 +1,14 @@
+package main
+
+import (
+	"fmt"
+
+	"example/user/testing/importpackage"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func main() {
+	fmt.Println(importpackage.ReverseRunes("!oG ,olleH"))
+	fmt.Println(cmp.Diff("Hello World", "Hello Go"))
+}
\ No newline at end of file
diff --git a/4-testing/importpackage/reverse.go b/4-testing/importpackage/reverse.go
new file mode 100644
index 0000000..743724a
--- /dev/null
+++ b/4-testing/importpackage/reverse.go
@@ -0,0 +1,12 @@
+// Package importpackage implements additional functions to manipulate UTF-8
+// encoded strings, beyond what is provided in the standard "strings" package.
+package importpackage
+
+// ReverseRunes returns its argument string reversed rune-wise left to right.
+func ReverseRunes(s string) string {
+	r := []rune(s)
+	for i, j := 0, len(r)-1; i < len(r)/2; i, j = i+1, j-1 {
+		r[i], r[j] = r[j], r[i]
+	}
+	return string(r)
+}
\ No newline at end of file
diff --git a/4-testing/importpackage/reverse_test.go b/4-testing/importpackage/reverse_test.go
new file mode 100644
index 0000000..7d4edd4
--- /dev/null
+++ b/4-testing/importpackage/reverse_test.go
@@ -0,0 +1,19 @@
+package importpackage
+
+import "testing"
+
+func TestReverseRunes(t *testing.T) {
+	cases := []struct {
+		in, want string
+	}{
+		{"Hello, world", "dlrow ,olleH"},
+		{"Hello, 世界", "界世 ,olleH"},
+		{"", ""},
+	}
+	for _, c := range cases {
+		got := ReverseRunes(c.in)
+		if got != c.want {
+			t.Errorf("ReverseRunes(%q) == %q, want %q", c.in, got, c.want)
+		}
+	}
+}
\ No newline at end of file
diff --git a/4-testing/readme.md b/4-testing/readme.md
new file mode 100644
index 0000000..4f50beb
--- /dev/null
+++ b/4-testing/readme.md
@@ -0,0 +1,36 @@
+# Testing
+Go has a lightweight test framework composed of the go test command and the testing package.
+
+Write a test by creating a file with a name ending in `_test.go` that contains functions named TestXXX with signature `func (t *testing.T)`. The test framework runs each such function; if the function calls a failure function such as t.Error or t.Fail, the test is considered to have failed. 
+
+```bash
+cp -r 3-importremote 4-testing
+# change importremote to testing
+nano go.mod
+nano hello.go
+# create test for reverse
+cd importpackage
+nano reverse_test.go
+>   package morestrings
+>   
+>   import "testing"
+>   
+>   func TestReverseRunes(t *testing.T) {
+>   	cases := []struct {
+>   		in, want string
+>   	}{
+>   		{"Hello, world", "dlrow ,olleH"},
+>   		{"Hello, 世界", "界世 ,olleH"},
+>   		{"", ""},
+>   	}
+>   	for _, c := range cases {
+>   		got := ReverseRunes(c.in)
+>   		if got != c.want {
+>   			t.Errorf("ReverseRunes(%q) == %q, want %q", c.in, got, c.want)
+>   		}
+>   	}
+>   }
+go build
+# run test code
+go test
+```
\ No newline at end of file
diff --git a/4-testing/testing b/4-testing/testing
new file mode 100755
index 0000000..ac40159
Binary files /dev/null and b/4-testing/testing differ