These set pieces just keep getting more and more amazing... as you go deeper into the volcanic area, it transitions from a wooden fort to a wooden roller coaster park to a factory (making polution just for the sake of making polution).... there's an incredible amount of variety in the environments. Also these navigation puzzles are getting really fun now too. All of the battles at this part of the game seem harder than before, with hopper enemies that cause an explosion where they land, and smasher enemies that have a wide area of damage... but I'm also much better at strategizing now so I'm zooming through most of it on the first try.
Deeper into the volcano, there's the midboss battle against Bowser Jr in his mech. This took me a few tries, he can do a ton of damage no matter where you try to hide... The strategy I came up with was to split apart my team to opposite sides, and have Rabbit Peach lure BJ into throwing his remote bomb toward her, so that she could shoot him and make him run towards her while running into his own bomb, and Rabbid Peach could shield and heal herself to stay alive while the others shoot at BJ when it's safe.
After this midboss is when you get the final party member, Yoshi. 4-6 is mostly about escorting Spawny to the end of the area (just like the escort Toad missions in earlier worlds). These are usually pretty easy... although when they added in those teleporting enemies it got kinda hectic.
Now we seem to be getting close to the end... Spawny gets captured by a bird that is the source of that ominous looking vortex in the sky...
Now we're back to doing actual hard battles... I didn't find Yoshi very useful, since his ground pound move requires him to position himself close to a bunch of enemies in order to do damage. Both Yoshi and Rabbid Yoshi don't get used much by me. But the rest of the Rabbid characters are very useful, and Peach and Luigi still get a lot of use out of me depending on the situation... This battle against Bwario and Bwaluigi wasn't anything special, they're basically just a ziggy and smasher, but they have more HP and longer range attacks (Bwaluigi can use Rabbid Mario's magnet ability which can be very tricky to avoid)
4-8 had a small gauntlet of ridiculously tough looking battles, Rabbid Luigi and Rabbid Mario proved to be way too useful. I think at this rate I could probably finish the game without going back to do the world 3 extra challenges for more upgrades.