Awesome!
I doubt this will be on the NX, even as a multiplat. Pikmin 3 started development on the Wii and used a few older textures in places, so it makes more sense for Nintendo to use the current assets rather than making the necessary next-gen investment. Especially since sales have never been that great for pikmin.
As for the release date, I doubt this will be like Pikmin 3 where it is announced six years early. Pikmin is developed by a lot of the same people as the New Super Mario Bros series, including Hiroyuki Kimura. NSMB became huge at just the wrong time for Pikmin 3's development. Kimura himself has not been Producer for any game since Pikmin 3, and EAD Group 4 has only released Super Mario Maker since then, so Pikmin 4 should not be too long from now. I'm thinking summer to late 2016, depending on when the NX and other games are released.
As for what the game could be like, I can see a few possibilities. They could continue building on Pikmin 3's formula, but that could be tricky considering how it is limited by the Food and Days mechanics. A successor to Pikmin 2, with unlimited time and more stuff to hunt down, could also be interesting. Both would benefit from a cooperative way of playing the main story. Then there's the online play missing from Pikmin 3, which would be great for the series.