diff --git a/internal/2016/DayFour/code.go b/internal/2016/DayFour/code.go
new file mode 100644
index 0000000..b61d7d2
--- /dev/null
+++ b/internal/2016/DayFour/code.go
@@ -0,0 +1,83 @@
+package dayfour
+
+import (
+	"advent-of-code/internal/registry"
+	"os"
+	"regexp"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+func init() {
+	registry.Register("2016D4", ParseInput, PartOne, PartTwo)
+}
+
+func ParseInput(filepath string) []string {
+	content, _ := os.ReadFile(filepath)
+	return strings.Split(string(content), "\n")
+}
+
+func isValidRoom(encryptedName string, expectedChecksum string) bool {
+	letterFrequency := [26]int{}
+
+	for _, character := range encryptedName {
+		if character >= 'a' && character <= 'z' {
+			letterFrequency[character-'a']++
+		}
+	}
+
+	type letterFrequencyPair struct {
+		letter rune
+		count  int
+	}
+
+	letterFrequencyPairs := make([]letterFrequencyPair, 0, 26)
+	for index, frequency := range letterFrequency {
+		if frequency > 0 {
+			letterFrequencyPairs = append(letterFrequencyPairs, letterFrequencyPair{
+				letter: rune('a' + index),
+				count:  frequency,
+			})
+		}
+	}
+
+	sort.Slice(letterFrequencyPairs, func(i, j int) bool {
+		if letterFrequencyPairs[i].count != letterFrequencyPairs[j].count {
+			return letterFrequencyPairs[i].count > letterFrequencyPairs[j].count
+		}
+		return letterFrequencyPairs[i].letter < letterFrequencyPairs[j].letter
+	})
+
+	if len(letterFrequencyPairs) < 5 {
+		return false
+	}
+
+	for index := range 5 {
+		if letterFrequencyPairs[index].letter != rune(expectedChecksum[index]) {
+			return false
+		}
+	}
+
+	return true
+}
+
+var roomPattern = regexp.MustCompile(`^(.+)-(\d+)\[([a-z]{5})\]$`)
+
+func PartOne(data []string) int {
+	sum := 0
+	for _, line := range data {
+		matches := roomPattern.FindStringSubmatch(line)
+		encryptedName := matches[1]
+		sectorIdentifier, _ := strconv.Atoi(matches[2])
+		actualChecksum := matches[3]
+		if isValidRoom(encryptedName, actualChecksum) {
+			sum += sectorIdentifier
+		}
+	}
+	return sum
+}
+
+func PartTwo(data []string) int {
+	return 0
+}