I think Metroid Prime 4 can at the very least emulate the success of the original Metroid Prime on the Gamecube. When MP launched people were hungry for a new Metroid game, the title looked phenomenal and unlike anything Nintendo had done before and it sold those 3 million on one of the worst selling Nintendo consoles ever.
If Retro Studios manages to make an excellent game and push the Switch on the technical side like they did with the original trilogy... I think success is granted. There is definitely hunger for a new big Metroid game and unlike the Gamecube, the Switch is immensely popular and sells like crazy. If Metroid Prime could sell 3 millions on the Gamecube... making the right moves Metroid Prime 4 can do that and more on the Switch. I'm gonna get crazy and say 5 million as long as Nintendo doesn't do anything stupid. Like launching the game when the Switch is not relevant anymore alla Samus Returns. If they do that, then 2-3 million tops.
EDIT: Oh, the floor. Like... the minimum. If everything goes well, 3 million. If they fuck it up, 1 million.