diff --git a/day4/main.go b/day4/main.go
new file mode 100644
index 0000000..a491165
--- /dev/null
+++ b/day4/main.go
@@ -0,0 +1,108 @@
+package main
+
+import (
+	"log"
+	"math"
+	"strconv"
+)
+
+func main() {
+	countPasswords := CountPossiblePasswords(245318, 765747, false)
+	countAdditionalPasswords := CountPossiblePasswords(245318, 765747, true)
+
+	log.Println("Part1: " + strconv.Itoa(countPasswords))
+	log.Println("Part2: " + strconv.Itoa(countAdditionalPasswords))
+}
+
+func CountPossiblePasswords(start int, end int, additional bool) int {
+	sum := 0
+	for i := start; i < end; i++ {
+		if MeetPasswordCriteria(i, additional) {
+			sum++
+		}
+	}
+	return sum
+}
+
+func MeetPasswordCriteria(number int, additional bool) bool {
+
+	if CountDigits(number) != 6 {
+		return false
+	}
+
+	if !AreDigitsNotDecreasing(number) {
+		return false
+	}
+
+	if !HaveDoubleInputs(number) {
+		return false
+	}
+
+	if additional && !HasOneDouble(number) {
+		return false
+	}
+
+	return true
+}
+
+func CountDigits(i int) int {
+	count := 0
+	for i != 0 {
+		i /= 10
+		count = count + 1
+	}
+	return count
+}
+
+func AreDigitsNotDecreasing(i int) bool {
+	iStr := strconv.Itoa(i)
+	prevInt := math.MinInt32
+
+	for _, a := range iStr {
+		aInt, _ := strconv.Atoi(string(a))
+
+		if prevInt > aInt {
+			return false
+		}
+		prevInt = aInt
+	}
+	return true
+}
+
+func HaveDoubleInputs(i int) bool {
+	iStr := strconv.Itoa(i)
+	hasDouble := false
+	for i := 0; i < len(iStr)-1; i++ {
+		if iStr[i] == iStr[i+1] {
+			hasDouble = true
+		}
+	}
+	return hasDouble
+}
+
+func HasOneDouble(i int) bool {
+	iStr := strconv.Itoa(i)
+
+	for len(iStr) >= 2 {
+		if iStr[0] != iStr[1] {
+			iStr = iStr[1:]
+			continue
+		}
+
+		i := 1
+		for len(iStr) > i && iStr[0] == iStr[i] {
+			i++
+		}
+
+		if i == 2 {
+			return true
+		}
+
+		if len(iStr) <= i {
+			return false
+		}
+
+		iStr = iStr[i:]
+	}
+	return false
+}
diff --git a/day4/main_test.go b/day4/main_test.go
new file mode 100644
index 0000000..88503f1
--- /dev/null
+++ b/day4/main_test.go
@@ -0,0 +1,156 @@
+package main
+
+import "testing"
+
+func TestCountDigitsPositiveNumber(t *testing.T) {
+	count := CountDigits(237841)
+	solution := 6
+
+	if count != solution {
+		t.Errorf("DigitCount was incorrect, got: {%d}, want: {%d}.", count, solution)
+	}
+}
+
+func TestCountDigitsNegativeNumber(t *testing.T) {
+	count := CountDigits(-2378541)
+	solution := 7
+
+	if count != solution {
+		t.Errorf("DigitCount was incorrect, got: {%d}, want: {%d}.", count, solution)
+	}
+}
+
+func TestAreDigitsNotDecreasingSuccess(t *testing.T) {
+	success := AreDigitsNotDecreasing(1134569)
+	solution := true
+
+	if success != solution {
+		t.Errorf("AreDigitsNotDecreasing was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestAreDigitsNotDecreasingFailure(t *testing.T) {
+	success := AreDigitsNotDecreasing(12246378)
+	solution := false
+
+	if success != solution {
+		t.Errorf("AreDigitsNotDecreasing was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestHaveDoubleInputsSuccess(t *testing.T) {
+	success := HaveDoubleInputs(123445)
+	solution := true
+
+	if success != solution {
+		t.Errorf("HaveDoubleInputs was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestHaveDoubleInputsFailure(t *testing.T) {
+	success := HaveDoubleInputs(12345)
+	solution := false
+
+	if success != solution {
+		t.Errorf("HaveDoubleInputs was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestHaveDoubleInputsSuccessAdditional(t *testing.T) {
+	success := HasOneDouble(114444)
+	solution := true
+
+	if success != solution {
+		t.Errorf("HaveDoubleInputs was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestHaveDoubleInputsSuccessAdditional2(t *testing.T) {
+	success := HasOneDouble(111444)
+	solution := false
+
+	if success != solution {
+		t.Errorf("HaveDoubleInputs was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestHaveDoubleInputsSuccessAdditional3(t *testing.T) {
+	success := HasOneDouble(111114)
+	solution := false
+
+	if success != solution {
+		t.Errorf("HaveDoubleInputs was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestHaveDoubleInputsSuccessAdditional4(t *testing.T) {
+	success := HasOneDouble(111111)
+	solution := false
+
+	if success != solution {
+		t.Errorf("HaveDoubleInputs was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestHaveDoubleInputsFailureAdditional(t *testing.T) {
+	success := HasOneDouble(1234555)
+	solution := false
+
+	if success != solution {
+		t.Errorf("HaveDoubleInputs was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestCheckPasswordCriteriaExample1(t *testing.T) {
+	success := MeetPasswordCriteria(111111, false)
+	solution := true
+
+	if success != solution {
+		t.Errorf("PasswordCriteria was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestCheckPasswordCriteriaExample2(t *testing.T) {
+	success := MeetPasswordCriteria(223450, false)
+	solution := false
+
+	if success != solution {
+		t.Errorf("PasswordCriteria was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestCheckPasswordCriteriaExample3(t *testing.T) {
+	success := MeetPasswordCriteria(123789, false)
+	solution := false
+
+	if success != solution {
+		t.Errorf("PasswordCriteria was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestCheckPasswordAdditionalCriteriaExample1(t *testing.T) {
+	success := MeetPasswordCriteria(112233, true)
+	solution := true
+
+	if success != solution {
+		t.Errorf("PasswordCriteria was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestCheckPasswordAdditionalCriteriaExample2(t *testing.T) {
+	success := MeetPasswordCriteria(123444, true)
+	solution := false
+
+	if success != solution {
+		t.Errorf("PasswordCriteria was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}
+
+func TestCheckPasswordAdditionalCriteriaExample3(t *testing.T) {
+	success := MeetPasswordCriteria(111122, true)
+	solution := true
+
+	if success != solution {
+		t.Errorf("PasswordCriteria was incorrect, got: {%t}, want: {%t}.", success, solution)
+	}
+}