According to Myamoto, Pikmin 4 was supposed to be very close to completion since 2015:
I think it's safe to assume that this game was probably being made for the Wii U, but then it started to be retooled for the Switch when Nintendo realised Wii U situation wasn't going to change, and that the game could have a better chance on a new console. But come on, it has been 3 years already, it can't possibly take that long to do that, can it? That's why I think they're not just retooling the game for Switch, but also adding many new things to it.
"lupus in fabula, venit enim ad me."
Translation: I will always hate Fox for canceling The Exorcist.