2) Bullet density and speed goes up with the difficulty, the time between enemy waves goes down, but waves always spawn early if there's nothing hostile alive because the game just has a lot of dead time otherwise. Patterns aren't actually unique per difficulty but circles/arcs have a smaller step between shots.
3) I was out of ideas for how to escalate the difficulty (probably for the better) so level 4 is just the final boss.
4) It's so bad I saw no point in releasing it (it's the first project I made without copying a Game Maker tutorial).