So far, I think my favorite parts of Pikmin 4 are the new areas, new enemies, Oatchi, and cave exploring. I missed exploring caves even more than I realized. The new types of caves are a breath of fresh air, are fun to explore, the lack of a time limit offers a nice break from the surface, the new mechanics introduced in the caves that I've discovered are fun so far, and the return and enhancements of some old level layouts and themes and asethetics from Pikmin 2 are a major blast from the past.
Pikmin 2's caves were a major feature, and while Pikmin 3's gameplay worked better without it (the larger areas, amount of extremely strong surface level bosses, and time limit based on your food supply probably made it for the best that caves didn't return) seeing them return in Pikmin 4 all new and improved is more than welcome return of one the series' best additions. Gameplay-wise, the game is as fun as it's predecessors. Task management and strategy are emphasized even more here, the new Pikmin are great, and it's fun trying to get as many tasks done as possible within the span of a day.
I'm fine with there being an early limit on how many Pikmin you can have on the surface since this presents more challenge to the strategies you can employ and upgrades can allow you to gradually return to having 100 Pikmin on the surface again, but the decision to limit the player to having only three types of Pikmin on the surface not only isn't consistent with any of the previous games, but feels entirely like fake difficulty and the in-universe explanation being that the Pikmin and Onions evolved to prevent it as a means to avoid extinction makes little sense and just comes off as the developers struggling to justify the change.
Also, the decision to never have enemies respawn on the surface still makes no sense. I'm unironically hoping that some of the DLC includes an update or option to have them respawn after two or three days like in the previous games.
I haven't come close to beating the game yet as a lot of other games on my backlog have taken priority until recently, but while I have a few nitpicks, the game is a lot of a fun so far and is as worthy of an addition to the series as the previous mainline games are. So far, at least, I can say that Pikmin 4 was well worth the wait.