diff --git a/day04.go b/day04.go
new file mode 100644
index 0000000..73d41c2
--- /dev/null
+++ b/day04.go
@@ -0,0 +1,123 @@
+package aoc2019
+
+import (
+	"io/ioutil"
+	"strconv"
+	"strings"
+
+	"github.com/pkg/errors"
+)
+
+func day4NumberToDigitSlice(n int64) []int {
+	var out []int
+	sn := strconv.FormatInt(n, 10)
+	for _, c := range sn {
+		d, err := strconv.Atoi(string(c))
+		if err != nil {
+			// This should not happen as the input is a guaranteed base-10 number
+			panic(err)
+		}
+		out = append(out, d)
+	}
+
+	return out
+}
+
+func day4IsValidPassword(n, min, max int64) bool {
+	sn := day4NumberToDigitSlice(n)
+
+	// It is a six-digit number.
+	if len(sn) != 6 {
+		return false
+	}
+
+	// The value is within the range given in your puzzle input.
+	if n < min || n > max {
+		return false
+	}
+
+	// Two adjacent digits are the same (like 22 in 122345).
+	var foundAdjacentMatch bool
+	for i := 1; i < len(sn); i++ {
+		if sn[i] == sn[i-1] {
+			foundAdjacentMatch = true
+		}
+	}
+	if !foundAdjacentMatch {
+		return false
+	}
+
+	// Going from left to right, the digits never decrease; they only ever increase or stay the same (like 111123 or 135679).
+	for i := 1; i < len(sn); i++ {
+		if sn[i] < sn[i-1] {
+			return false
+		}
+	}
+
+	return true
+}
+
+func day4IsValidPasswordPart2(in, min, max int64) bool {
+	// Previous rules still apply
+	if !day4IsValidPassword(in, min, max) {
+		return false
+	}
+
+	sn := day4NumberToDigitSlice(in)
+
+	var (
+		last  = sn[0]
+		count = 1
+	)
+
+	for i := 1; i < len(sn); i++ {
+		if sn[i] != last && count == 2 {
+			return true
+		}
+
+		if sn[i] == last {
+			count++
+			continue
+		}
+
+		last = sn[i]
+		count = 1
+	}
+
+	return count == 2
+}
+
+func solveDay4WithFunction(inFile string, vf func(int64, int64, int64) bool) (int, error) {
+	raw, err := ioutil.ReadFile(inFile)
+	if err != nil {
+		return 0, errors.Wrap(err, "Unable to read input file")
+	}
+
+	pts := strings.Split(strings.TrimSpace(string(raw)), "-")
+	min, err := strconv.ParseInt(pts[0], 10, 64)
+	if err != nil {
+		return 0, errors.Wrap(err, "Unable to parse minimum")
+	}
+
+	max, err := strconv.ParseInt(pts[1], 10, 64)
+	if err != nil {
+		return 0, errors.Wrap(err, "Unable to parse maximum")
+	}
+
+	var count int
+	for i := min; i <= max; i++ {
+		if vf(i, min, max) {
+			count++
+		}
+	}
+
+	return count, nil
+}
+
+func solveDay4Part1(inFile string) (int, error) {
+	return solveDay4WithFunction(inFile, day4IsValidPassword)
+}
+
+func solveDay4Part2(inFile string) (int, error) {
+	return solveDay4WithFunction(inFile, day4IsValidPasswordPart2)
+}
diff --git a/day04_input.txt b/day04_input.txt
new file mode 100644
index 0000000..41f9cf1
--- /dev/null
+++ b/day04_input.txt
@@ -0,0 +1 @@
+125730-579381
diff --git a/day04_test.go b/day04_test.go
new file mode 100644
index 0000000..94ecc9a
--- /dev/null
+++ b/day04_test.go
@@ -0,0 +1,58 @@
+package aoc2019
+
+import (
+	"math"
+	"reflect"
+	"testing"
+)
+
+func TestDay4NumberToDigitSlice(t *testing.T) {
+	for n, expSlice := range map[int64][]int{
+		1234567:  {1, 2, 3, 4, 5, 6, 7},
+		43626145: {4, 3, 6, 2, 6, 1, 4, 5},
+	} {
+		if s := day4NumberToDigitSlice(n); !reflect.DeepEqual(expSlice, s) {
+			t.Errorf("Number to slice for number %d yield unexpected result: exp=%+v got=%+v", n, expSlice, s)
+		}
+	}
+}
+
+func TestDay4ValidPassword(t *testing.T) {
+	for n, expValid := range map[int64]bool{
+		111111: true,
+		223450: false,
+		123789: false,
+	} {
+		if v := day4IsValidPassword(n, 0, math.MaxInt64); v != expValid {
+			t.Errorf("Number %d did not have expected validity: exp=%v got=%v", n, expValid, v)
+		}
+	}
+
+	for n, expValid := range map[int64]bool{
+		112233: true,
+		123444: false,
+		111122: true,
+	} {
+		if v := day4IsValidPasswordPart2(n, 0, math.MaxInt64); v != expValid {
+			t.Errorf("Number %d did not have expected validity for part 2: exp=%v got=%v", n, expValid, v)
+		}
+	}
+}
+
+func TestCalculateDay4_Part1(t *testing.T) {
+	count, err := solveDay4Part1("day04_input.txt")
+	if err != nil {
+		t.Fatalf("Day 4 solver failed: %s", err)
+	}
+
+	t.Logf("Solution Day 4 Part 1: %d", count)
+}
+
+func TestCalculateDay4_Part2(t *testing.T) {
+	count, err := solveDay4Part2("day04_input.txt")
+	if err != nil {
+		t.Fatalf("Day 4 solver failed: %s", err)
+	}
+
+	t.Logf("Solution Day 4 Part 2: %d", count)
+}