super fun brain-bender. I've played games where I have to keep track of two units at a time, but four is very difficult. I'd have to say your strongest feature was introducing new elements to the player and then later combining them for a challenge! It was very simple and intuitive. My only issue with the game was that certain backgrounds were a little too dark for me to see clearly, and some of the objects would occasionally become indistinguishable from each other--I don't think the the focus of the game is to make players struggle to see their environment, they should be using all their effort to solve the puzzles. I would recommend brightening them just a tad.
I love puzzling games like these. If you choose to continue making some levels, maybe you could do some hardcore-mode ones where players keep track of five or even six units at a time!