Edit: FUCK THIS SHIT!!!! I was logged out during my long review message and have to write it all over again.
Damnage and Damnation by XutaWoo and Pyro
Ew, These Non-Newtonian Fluids are Orange! by Henry Richards
ATTACK OF THE SLIME MONSTERS by Pyro
Finally I have one more suggestion for SAJewers.
Since you have added a few open spots to the different worlds I'd actually add 1 to 2 more levels to world 4 as it feels very short compared to the other worlds.
It wouldn't even need new levels to be made since the two levels I have in mind as perfect candidates for world 4 are already done.
I'd put both Damnage and the Non-Newtonian Fluids level in this world.
With the update and the additional checkpoints Damnage is a very fair level and is actually easier than a few of the otehr world 4 levels like Haizy Maze Cave or Bat-an-Eye. It would fit perfectly in a world with some rather short levels. For world 9 it's way too easy right now. But as a world 4 level it would just work fine.
The Fluids level would also fit into world 4.
I now realized that Damnage and Damnation was supposed to be the penultimate level of world 9? Like really? As I wrote it's not that difficult, world 4 difficulty at the very least. Most of the world 9 levels if not all of them are way, way harder than this: just compare it to Super Demo and Needle Cyclops. When it comes to a perfect penultimate level I'd pick the level by Franky (which is really unforgiving) or the Luna Tower sequel (which is long and really hard). Both of these would be better candidates. I don't judge world 9 by theme but by difficulty and when it comes to Damnation it is a fantastic level, but it just isn't world 9 stuff.
So SAJewers, I suggest playing other world 4 levels again quickly (especially the harder ones like Bat-an-Eye or Haizy Maze Cave) and compare it to those. You will probably notice that it's not that much harder (I'd even dare to say that it's easier than the Bat level).
Haizy Maze Cave puts the player under some pressure due to the poison filled cave an when you run to the item that protects you from it you ahve to deal with a lot of enemies). Damnation however doesn't have a lot of enemies on screen. Everything is very easy to plan ahead. So it really feels like world 4.
So yeah putting these two levels in this world would make it as long as world 6 and offer a nice variety of awesome cave levels.