^^
We are talking about a story driven, campaign based, 3rd person shooters right? I only know of 3 games like this on 360.

Gears of War
Gears of War 2
Army of 2

Army of 2 was a mediocre game and it got over 1.5 mil in sales and is a new IP.
Resident Evil 5 from the looks of things (demo) will be the best looking 360 game to date, and a AAA game...and is an established franchise with guaranteed million sales even if the game gets 0/10 on metacritic.

Predicting 1-2 million for the 360 version is just being irrational IMO.

"We are talking about a story driven, campaign based, 3rd person shooters right? I only know of 3 games like this on 360."

You can't base sales predictions on broad assumptions of the genre of a game. Things like marketing and whether enough people actually want the game (or are convinced they want it by the marketing) matter more.

Those are not known, thus assuming 4 million copies sold is not a given.