diff --git a/chapter_04/activity_4.06/main.go b/chapter_04/activity_4.06/main.go
new file mode 100644
index 0000000..ff85f01
--- /dev/null
+++ b/chapter_04/activity_4.06/main.go
@@ -0,0 +1,41 @@
+package main
+
+import "fmt"
+
+func getData() []interface{} {
+
+  return []interface{}{
+    1,
+    3.14,
+    "hello",
+    true,
+    struct{}{},
+  }
+
+}
+
+func typeCheck(v interface{}) string {
+
+  switch v.(type) {
+  case string:
+    return "string"
+  case bool:
+    return "bool"
+  case float32, float64:
+    return "float"
+  case int, int32, int64:
+    return "int"
+  default:
+    return "unknown"
+  }
+
+}
+
+func main() {
+
+  data := getData()
+  for i := 0; i < len(data); i++ {
+    fmt.Printf("%v is %v\n", data[i], typeCheck(data[i]))
+  }
+
+}
diff --git a/chapter_04/exercise_4.18/main.go b/chapter_04/exercise_4.18/main.go
new file mode 100644
index 0000000..9017ec1
--- /dev/null
+++ b/chapter_04/exercise_4.18/main.go
@@ -0,0 +1,40 @@
+package main
+
+import "fmt"
+
+type point struct {
+  x int
+  y int
+}
+
+func compare() (bool, bool) {
+
+  point1 := struct {
+    x int
+    y int
+  }{
+    10,
+    10,
+  }
+
+  point2 := struct {
+    x int
+    y int
+  }{}
+  point2.x = 10
+  point2.y = 5
+
+  point3 := point{10, 10}
+
+  return point1 == point2, point1 == point3
+
+}
+
+func main() {
+
+  a, b := compare()
+
+  fmt.Println("point1 == point2:", a)
+  fmt.Println("point1 == point3:", b)
+
+}
diff --git a/chapter_04/exercise_4.19/main.go b/chapter_04/exercise_4.19/main.go
new file mode 100644
index 0000000..2a0d9e6
--- /dev/null
+++ b/chapter_04/exercise_4.19/main.go
@@ -0,0 +1,65 @@
+package main
+
+import "fmt"
+
+type name string
+
+type location struct {
+  x int
+  y int
+}
+
+type size struct {
+  width  int
+  height int
+}
+
+type dot struct {
+  name
+  location
+  size
+}
+
+func getDots() []dot {
+
+  var dot1 dot
+
+  dot2 := dot{}
+  dot2.name = "A"
+  dot2.x = 5
+  dot2.y = 6
+  dot2.width = 10
+  dot2.height = 20
+
+  dot3 := dot{
+    name: "B",
+    location: location{
+      x: 13,
+      y: 27,
+    },
+    size: size{
+      width:  5,
+      height: 7,
+    },
+  }
+
+  dot4 := dot{}
+  dot4.name = "C"
+  dot4.x = 101
+  dot4.location.y = 209
+  dot4.width = 87
+  dot4.size.height = 43
+
+  return []dot{dot1, dot2, dot3, dot4}
+
+}
+
+func main() {
+
+  dots := getDots()
+
+  for i := 0; i < len(dots); i++ {
+    fmt.Printf("dot%v: %#v\n", i+1, dots[i])
+  }
+
+}
diff --git a/chapter_04/exercise_4.20/main.go b/chapter_04/exercise_4.20/main.go
new file mode 100644
index 0000000..5ead87f
--- /dev/null
+++ b/chapter_04/exercise_4.20/main.go
@@ -0,0 +1,26 @@
+package main
+
+import (
+  "fmt"
+  "math"
+)
+
+func convert() string {
+
+  var i8 int8 = math.MaxInt8
+  i := 128
+  f64 := 3.14
+  m := fmt.Sprintf("int8    =  %v -> int64   =  %v\n", i8, int64(i8))
+  m += fmt.Sprintf("int     =  %v -> int8    = %v\n", i, int8(i))
+  m += fmt.Sprintf("int8    =  %v -> float32 =  %v\n", i8, float64(i8))
+  m += fmt.Sprintf("float64 = %v -> int     =    %v\n", f64, int(f64))
+
+  return m
+
+}
+
+func main() {
+
+  fmt.Print(convert())
+
+}
diff --git a/chapter_04/exercise_4.21/main.go b/chapter_04/exercise_4.21/main.go
new file mode 100644
index 0000000..b13d901
--- /dev/null
+++ b/chapter_04/exercise_4.21/main.go
@@ -0,0 +1,29 @@
+package main
+
+import (
+  "errors"
+  "fmt"
+)
+
+func doubler(v interface{}) (string, error) {
+
+  if i, ok := v.(int); ok {
+    return fmt.Sprint(i * 2), nil
+  }
+  if s, ok := v.(string); ok {
+    return s + s, nil
+  }
+  return "", errors.New("unsupported type passed")
+
+}
+
+func main() {
+
+  res, _ := doubler(5)
+  fmt.Println("5 :", res)
+  res, _ = doubler("yum")
+  fmt.Println("yum:", res)
+  _, err := doubler(true)
+  fmt.Println("true:", err)
+
+}
diff --git a/chapter_04/exercise_4.22/main.go b/chapter_04/exercise_4.22/main.go
new file mode 100644
index 0000000..3ca2fa7
--- /dev/null
+++ b/chapter_04/exercise_4.22/main.go
@@ -0,0 +1,62 @@
+package main
+
+import (
+  "errors"
+  "fmt"
+)
+
+func doubler(v interface{}) (string, error) {
+
+  switch t := v.(type) {
+  case string:
+    return t + t, nil
+  case bool:
+    if t {
+      return "truetrue", nil
+    }
+    return "falsefalse", nil
+  case float32, float64:
+    if f, ok := t.(float64); ok {
+      return fmt.Sprint(f * 2), nil
+    }
+    return fmt.Sprint(t.(float32) * 2), nil
+  case int:
+    return fmt.Sprint(t * 2), nil
+  case int8:
+    return fmt.Sprint(t * 2), nil
+  case int16:
+    return fmt.Sprint(t * 2), nil
+  case int32:
+    return fmt.Sprint(t * 2), nil
+  case int64:
+    return fmt.Sprint(t * 2), nil
+  case uint:
+    return fmt.Sprint(t * 2), nil
+  case uint8:
+    return fmt.Sprint(t * 2), nil
+  case uint16:
+    return fmt.Sprint(t * 2), nil
+  case uint32:
+    return fmt.Sprint(t * 2), nil
+  case uint64:
+    return fmt.Sprint(t * 2), nil
+  default:
+    return "", errors.New("unsupported type passed")
+  }
+
+}
+
+func main() {
+
+  res, _ := doubler(-5)
+  fmt.Println("-5:  ", res)
+  res, _ = doubler(5)
+  fmt.Println("5:   ", res)
+  res, _ = doubler("yum")
+  fmt.Println("yum: ", res)
+  res, _ = doubler(true)
+  fmt.Println("true:", res)
+  res, _ = doubler(float32(3.14))
+  fmt.Println("3.14:", res)
+
+}