feat: register day four

internal/2021/register.go

@@ -1,6 +1,7 @@
 package year2021
 
 import (
+	_ "advent-of-code/internal/2021/DayFour"
 	_ "advent-of-code/internal/2021/DayOne"
 	_ "advent-of-code/internal/2021/DayThree"
 	_ "advent-of-code/internal/2021/DayTwo"