Prepare day 4 and template.

src/main.rs

@@ -8,6 +8,7 @@ fn main() {
         (1, day01::day_main as fn()),
         (2, day02::day_main),
         (3, day03::day_main),
+        (4, day04::day_main),
     ]);
     let day: Option<u8> = args().nth(1).and_then(|a| a.parse().ok());
     let Some(day) = day else {