diff --git a/2015/days/day4.php b/2015/days/day4.php
new file mode 100644
index 0000000..3cf9464
--- /dev/null
+++ b/2015/days/day4.php
@@ -0,0 +1,21 @@
+<?php
+
+function day4(int $part = 1) {
+  $key = 'bgvyzdsv';
+  $stop_value = 0;
+  for ($i = 0; $i < 10000000; $i++) {
+    $stop_value = $key . $i;
+    $md5 = md5($stop_value);
+    if ($part == 1) {
+      $identifier = '00000';
+    }
+    elseif ($part == 2) {
+      $identifier = '000000';
+    }
+    if (str_starts_with($md5, $identifier)) {
+      break;
+    }
+  }
+  return str_replace($key, '', $stop_value);
+}
+
diff --git a/2015/solutions.php b/2015/solutions.php
index 20d1d37..d65952b 100644
--- a/2015/solutions.php
+++ b/2015/solutions.php
@@ -12,6 +12,8 @@ $solutions['day_2']['part_1'] = day2(1);
 $solutions['day_2']['part_2'] = day2(2);
 $solutions['day_3']['part_1'] = day3(1);
 $solutions['day_3']['part_2'] = day3(2);
+$solutions['day_4']['part_1'] = day4(1);
+$solutions['day_4']['part_2'] = day4(2);
 
 // OUTPUT THE SOLUTIONS WE HAVE.
 foreach ($solutions as $day => $parts) {