diff --git a/4/script.rb b/4/script.rb
new file mode 100755
index 0000000..bce660e
--- /dev/null
+++ b/4/script.rb
@@ -0,0 +1,28 @@
+#!/usr/bin/env ruby
+
+require_relative '../common'
+
+RANGE = '372037-905157'
+
+def part1
+  first, last = RANGE.split('-')
+  (first..last).count do |password|
+    password = password.to_s
+    password.chars == password.chars.sort &&
+      password.match?(/(.)\1/)
+  end
+end
+
+def part2
+  first, last = RANGE.split('-')
+  (first..last).count do |password|
+    password = password.to_s
+    password.chars == password.chars.sort &&
+      password.chars.group_by(&:itself).any? { |_,v| v.size == 2 }
+  end
+end
+
+puts "=== Part 1 ==="
+puts part1
+puts "=== Part 2 ==="
+puts part2