I said 4 million.
The best selling system that had a Prime-ready install base was Gamecube at 21 million total sales. Metroid has always been a very well regarded but niche series and the Wii had a userbase that consisted of a lot of people who would never even think of trying out Metroid, plus after just having had two Metroid Prime's on GC perhaps a decent amount of core gamers on the Wii just didn't bother with what felt like a lot of Metroids coming out. I had Wii and am a Metroid fan but didn't get 3 while I get 1 and 2 on GC. But now after well over a decade of waiting, with the Prime series very highly regarded, with Switch's high gamer base in which core Nintendo games are selling through the roof, I think Prime 4 is gonna blow away every other Metroid.
So yeah I think 2.5 million in US, 1.5 million in Europe, and 500k in Japan and the RotW is the minimum it'll sell, and wouldn't be surprised to see it launch holiday 2021, sell like 3+ million that quarter alone, and work its way up to something like 7 million eventually.