Ease up on the PS2 is "the ultimte game seller" bandwagon.
1) More competition on the PS2 (simply put, more games on the shelves).
I mean, there were no released GC games for the month of April. Basically, RE4 has become the 'game' to get because it's almost literally, 'the only game to get.' Every month for the GC is like a launch month in ways. As in most companies like to release launch games because of the lack of competition. And RE4 (cube) GOT A LOT OF advertising. Capcom probably won't be doing the same for the PS2. In fact, other than major press ads (ads in mags like PSM, IGN) there probably won't be any other for the PS2 version.
2) MUCH, MUCH, MUCH more piracy on PS2. 999x's worse I'd say matey
3) More casuals own the PS2. Not saying it's a casual system, but it's quite obvious that PS2's "higher installed" base is from people who don't even buy games at all (i.e, interested in the console for the ability to pirate it). Especially if you inlcude the intall base for countries like Korea and China. And how many people bought a PS2 just for the DVD, and how many are 2nd through sales (as in your 1st PS2 broke on you).
4) A LOT OF you overblow Resident Evil's popularity. It's not that popular to sell 3 million.
And as far as I know, RE:Code Vernocia never hit 3 million (800,000 US) (400,000 JPN), and that was RE's FIRST JUMP TO NEXT-GEN consoles.
I highly doubt RE4 will do more on the PS2. With REmake and RE0 making debuts on the GC and the GC being a console where you can buy for 60 bucks (used), I think majority that are even interested in RE4 have already bought RE4. (warning: generalization) as in majority OUTSIDE of the 300 or so posters at gaming-age.