Added day 4.. early I know! And i fixed unit tests not compiling

src/main.rs

@@ -10,6 +10,7 @@ fn main() -> Result<(), Box<dyn Error>> {
         Box::new(day01::Day01 {}),
         Box::new(day02::Day02 {}),
         Box::new(day03::Day03 {}),
+        Box::new(day04::Day04 {}),
     ];
 
     // Run through and generate solutions