GTA4 IS a shooter. A third person shooter. It's part of a unique sub-subgenre (sandbox), but it's still a game in third person where gun combat is a primary function of the game. That makes it a shooter, in the most general sense of the word. Stealth games are the same if they have guns as a focus. Unless you can stealth through the entire game, which I don't believe you can do in MGS, or there are no guns.
Tomb Raider would NOT qualify as a shooter because the gun combat is a small secondary element of the game.
People need to realise, "shooter" is a VERY generalised term, and it can apply to vast amount of games.
Will you teach me to football?