Metal Gear Solid is kind of ( notice - KIND OF ) a niche game in Japan. Niche games are irrelevant to the platform's userbase, i.e. you are a Yu-Gi-Oh! fan and it's a Nintendo DS exclusive, you'll get it no matter what.
In conclusion, I think MGS4 will eventually reach the previous installment's sales on PS2 and slightly surpass them for a number of reasons :
1) Lack of competition ( by the time MGS3 launched, competition was fierce on PS2 )
2) PS3 userbase is more " hardcore " than PS2's.
Won't make it to 1million though. I'd be seriously amazed if it did.
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