The wings power-up is in level 4-1.
I was able to fix the graphical glitches introduced by my reworked Red Piranha Plant code pretty easily. However, while testing that, I noticed two other things about the Red Piranha Plants that needed fixing. Firstly, I renabled the code that I thought would make the Red Plants stay in their pipes when Mario is nearby, but it was working totally backwards. Rather than not come out, they'd not go in instead. This was as a result of the fact that they where moving in the opposite cycle as the Green Plants.
This also lead to the other issue I noticed. The plants where not syncronized properly. When a Green Plant and a Red Plant are near each other, they ought to move out of the pipe and and go back in at the same time. (Provided, of course, their cycle hasn't been disrupted by "hiding".) Instead one moves out while the other moves in.
To fix all of this, I've basically rewritten the Piranha Plant intialization code. Based on code from the original game, frantik's rewritten Green Piranha Plant code, and the Red Piranha Plant code from an unknown Japanese hacker, I've created a cleaned up implimentation of both plants types. However, I had to go to sleep last night before it was completely finished.
I should have the modifications complete sometime tonight. Once I get that done, I'll release a new expansion patch that incorportates the new plant code, worlds 1-7, and the bug fixes in levels in 1-4.
Once I'm done with that, I plan to look at fixing the graphics on the bottoms of the inverted pipes. While the fact that they're upside down is easy to ignore while playing the game, they stick out like a sore thumb in a sreenshot. Additionally, all this Piranha Plant hacking has given me a pretty good idea how to impliment new enemies in the SMB engine, so I'll look into that as well.