I went with 4 million. I really think that Metroid Prime gamers are starved for the series right now and Retro Studios gamers are as well after playing and enjoying the Donkey Kong Country game. There are so many out there just itching to play this title that even an 80 on Metacritic won't keep them away. While the Metroid and the Prime series don't have the greatest of sales, I think that many, many more people have played these games via used or borrrowed copies and that the audience is much larger than it appears. I also think that the Switch just defies all odds when it comes to software sales and in an ecosystem that is sorely lacking quality, new FPS titles, MP4 will be something that will fill a very large void.
I think 2 million at launch should be expected and that the legs on this title will easily take it to 4 million and it will join the pantheon of Switch titles that have turned better numbers than all previous iterations in their respective franchises.