Yeah, MGS2 and GTAIII really drove the Playstation 2's mainstream success at the end of '01. The 3 has had some OK games, but nothing to really blow the socks off people and make them change the way they see gaming like those two did.
MGS4 maybe could, but the mainstream gamer has changed since 1998, when MGS1 came out. And MGS2 really put Joe Donuts Playstation off with its weird story, and not nearly as many played MGS3 even with a huge userbase. The other big games are multiplatform.
You can compare the two systems' starts with numbers, but I think the reality is a lot different than just looking at a stats sheet (considering competition, public perception, DVD vs. BR, etc.)
The sales of MGS have indeed dropped, but I think MGS4 could be the series' revival in terms of sales and sheer impressiveness. (is that even a word?!) It does appear to have awfully deep gameplay and a more digestable story set in a very well timed location (people are mad about warring in the desert now, it seems).
Anyway; we'll see. I personally have very high expectations for it!