Its so impressive when someone manages to make a compelling game without much graphics or music. Just rectangles and gameplay and yet it manages to be fun.
I'd say level 3 was perfect difficulty. Initially felt way too hard, failed a bunch, then it suddenly clicked and I could easily do it which was satisfying.
But level 4 crossed the line for me. I tried quite a few times and couldn't do it. Fine for a game with only 5 levels, but if you do a longer version I'd ramp up the difficulty much slower than that.