It's time to bump this thread for hopefully the last time, because it's October and I've resumed working on this hack to finish the 2 remaining worlds!
Here are screenshots of each level of world 4. Note that this world currently doesn't have a name.
This level has no new enemies, but Bullet Bills make come back in this level again. Dry Bones are also present.
Ignore the timer set to 0, as that has been fixed.
Level 2 (Ghost House):
In this level, there's a Boo Stream that constantly chases Mario.
This level has a Boo that acts similar to the one of the previous level, except it's not a stream, but it can spit fireballs!
Also note, this level and the subsequent ones don't have the darkness HDMA present yet. They will be added later.
The background is the Labyrinth Zone background
from the Sonic 1 protoype, but without the light things.
Doppelshrooms come back in this level, and it also introduces the Mario-solid and sprite-solid blocks. The enemies also are transcluent due to this level making use of a dark BG level mode.
The background is the underground background
from the All-Stars version of Super Mario Bros. 2.
This level has no new enemies, but there are Note Blocks.
Note that the palette of the wooden pillars have been fixed after taking these screenshots.
The background is this one
that was made by Gamma V.
This level makes use of Thwomps and Mad Thwomps, alongside the Mario-solid and sprite-solid blocks.
The Thwomps also got new graphics.