The legendary Shigeru Miyamoto has just confirmed that Pikmin 4 is in development in an interview, and he also confirmed that it is close to being finished. He didn't reveal anything beyond that, but it is probably a safe assumption to think that it will be released for their next console NX unless it is being prepared as a swan song of sorts for Wii U. And perhaps we shouldn't rule out a portable Pikmin for Nintendo 3DS.
"We can confirm that Pikmin 4 is in development but that is all we can confirm at present. It's actually very close to completion. Pikmin teams are always working on the next one."