According to Myamoto, Pikmin 4 was supposed to be very close to completion since 2015:

https://www.eurogamer.net/articles/2014-07-20-pikmin-4-in-development-and-very-close-to-completion

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.

I only hope it doesn't come to us as "Pikmin 4: Pik Pik Party Pak!" a mario party style board game

Given that we have DK coming soon then i don't expect more then 2 medium-big Nintendo games this holiday.

If it does get announced it would be for q2 2019.

I think there is a really big chance we will see Pikmin 4 this year and if we do it will probably be in the last big Direct of the year. I think the game will release  summer 2019, either July or August

Since when?

I am not sure what they will make from pikmin 4, but one thing i am sure is that i can’t play it without IR pointer controls!

My bad it already released. 

This holiday would be the perfect time to launch it. Along with Smash, a possible Animal Crossing, and two other support franchises such as Yoshi and Bayonetta, Pikmin would be a great August/Early September release.

I'm probably 85% certain it will be shown this E3. I have no proof, but I believe it is likely.

It seems as though they've been sitting on this game for a hot minute...
It occurred to me when they announced Hey Pikmin that Hey Pikmin was actually the game to which they were referring, but I don't think we have any reason to suspect that is the case...

I am actually beginning to wonder if this is not going to be one of their mobile games. Pikmin has a good setup for a micro-transaction type mobile game.

If the game is Switch-bound Miyamoto said that the game was near completion in 2015 something which he reiterated in 2017. It likely that when the games development started the game was targeting release on Wii U. After the Wii U did not pan out, they probably withheld release until Switch came out and mostly sitting (or who knows maybe even revamping it to make it more mainstream).