Added Level 4

After a long time  I added a fourth level to the game. This is a smaller level than the previous ones. I also made several fixes to the gameplay in the previous level, should be better now but there still room for more improvement. Level 2 has a weird bug where tux dies for no apparent reason near the end. In Level 3 and 4 tux is to big in comparison with level 1 and 2. That needs to be fixed in the future.