Snake in MGS4 is OLD SNAKE, the word SOLID SNAKE minus IS (being). FOXDIE, created to kill him, mutates into a weapon that kills the world in Snake's womb. The man who saved the world becomes a threat to the world in the end. This is superimposed on the BIG BOSS.
One last thing: Snake was taught everything about fighting by The Boss in MGS3, and at the end of MGS4, BIG BOSS embraces Snake with the CQC that The Boss taught him, the CQC that defeated him.