I dug the new level 4 as well. The "wheel" bumpers are fun since you get more ball interaction with them, and there's less room for the ball to fall between at first.
I think once you reach level 5 it just keeps repeating that level, no?
Can't wait for more levels, effects, modes, etc. I have lots of ideas for this, but don't know if you have time/interest to implement. For example a powerup that actually bumps/moves the bumpers when the ball hits it (they would just move once, not become additional balls and bounce around), or a powerup like Peggle has where it shows you the projected n bounces from your current firing direction (really helps in tricky shots).
Also it would be nice to have more options to customize the game experience. I don't know if the position of bumpers is fixed and has to be created in a level editor, but I'd love to be able to just turn up a dial and get more green, more red, more blue, etc. bumpers to customize game difficulty, or at least have a difficulty slider that adjusted these ratios for you (e.g. turning difficulty down would give you less red, more green, and maybe more balls).
I can also imagine a fun game mode with harder difficulties where you could store up powerups and choose to activate them at a particular time using hotkeys. Maybe this is overkill or going beyond the scope of the game. But I really think this is a great opportunity to take the Peggle-like gameplay and extend it to some innovative concepts.
Thanks again for all the hard work on this one. It's addictive and fun.