Pikmin 4's systems are not complicated. What changed is that the systems ask you to think in two teams instead of one, and the game's difficulty is tuned down accordingly.
Oatchi commands his own Pikmin squad. He rushes through breakable obstacles, jumps ledges, swims - which lets non-Blue Pikmin cross water - sniffs out treasure and castaways, carries objects, and can carry you. In practice this doubles your parallel actions: you can be moving a rock Pikmin squad at a crystal wall while Oatchi rushes a separate passage and opens a route for you. Learning to think of the two of you as two independent puzzle solvers is the skill the game actually teaches.
The obstacle list is the classic Pikmin ladder with a few new shapes. Dirt walls, electric gates, crystal and glass walls, ice walls, roots and potted plants, bridges, high ledges, cardboard boxes, paper bags, hydro jelly, cobblestone, mud pools, iron fences, mold, Kingcaps and clipboards all appear. Electric gates need Yellow Pikmin - or Oatchi with the Anti-Electrifier, or bomb rocks, or the co-op pebble pitcher. Crystal walls need Rock Pikmin. Ice walls need Ice Pikmin or fire starters.
Only three types can be outside at once, and only three can be taken into a cave. Finding Wild Pikmin raises those limits, and Flarlic gear adds ten to the field cap with a ceiling of 100. With the Fierce creature activity level enabled, the maximum squad drops to 60 - which is a real constraint on the harder modes.
Instead of replaying a day, Pikmin 4 lets you rewind. It replaces the error-correction loop of Pikmin 3 and, together with the absence of a day limit on main exploration, is the reason the game reads as relaxed rather than punishing. The charge throw - a locked-on charged throw carried over from Pikmin 3 - is unlocked later in the story rather than available from the start.
Two players can share a Switch, but the second player does not get a second captain. They control an on-screen pointer and throw pebbles or cover fire. It is a meaningful assist, and one of the most-cited weaknesses of the release - co-op is meaningfully weaker than it was in Pikmin 3 Deluxe. The Switch 2 Edition changes this, and also adds local wireless and network play.