In my opinion Stage 3 is possibly the worst stage that I've seen come from you.
The stage itself is just too bland and uncreative, literally the whole stage is the same obstacle course, with no variation or innovation. You could cut the stage out completely and the hack wouldn't lose much.
Stage 4 is kinda neat, there's some slowdown in the end that I would advise fixing. The shell tricks were creative and you managed to make them not too repetitive (wich is the problem with Stage 3).
I think that the main problem in your designs is the constant use of the same techniques. A level usually revolves around a mechanic, with tricks involving said mechanic, your levels usually revolve around one specific trick that you use over and over again, with little variation.
For some tips on how to make Stage 3 less repetitive, try this:
-On the entire stage you carry the shell, but you there's nothing trying to take it away from you, put some more enemies that can kill shells in contact, like bullet bills instead of eeries.
-You could also have some more uses to the shell instead of just at the end. Hiting turnblocks that block your way, killing some enemy that blocks your way that can only be killed by a shell, etc, and after that get the shell again before it falls of the screen or it hits one of those red kill-everything blocks.
Listening to metal, loudly.