MGS3 was the begining of the Story. It showed the origin of Big Boss. Big Boss had 3 clones later on: Solid [the main character in MGS1 and 4], Liquid [Villian in MGS1, dead] and Solidus [The main villian in MGS2, you also see him as the "Fake Big Boss" in the Van that was being held by Eve and her group in MGS4]. Raiden was introduced in MGS2 and is also the damn cool Cyborg like ninja who losses both his arms in MGS4. Raiden was a child soldier. You'll have to play MGS1 to get to know who Naomi is a little better.
The person that Snake fights at the end of MGS4 was Ocelot [the old man with the revolver in MGS2, and the young man with 2 revolver's in MGS3 that goes by the name Adamska or Adam.] The reason why he was called Liquid was because he pretended to be possessed by liquids arm.
EDITed
4 ≈ One
