Checkpoints seem to work. Tomorrow, I'm going to set them up in every level, play through the game, and see if it works well. I also will have to make some design changes to the end of Level 3.
I'm also going to postpone work on Level 5 until I finish a scoring system. I know some of you have been asking for this, and I realize now that this will be the best way to add some replay value to the game. Also, it will give incentive to not just run past everybody.
I did think of what the level 5 boss will be. I'm not going to say much (I like to wait to show a screenshot), but I will say one thing: it's big.