To be honest...I'm not sure it really matters all that much. If someone hadn't said something, I'd have never even noticed that they were different people (granted, that's namely due to the time that has passed; I suppose I'd probably have noticed if I saw Season 1 and then Season 4 right away...).
EDIT: Shit, and if the post under me is to be believed (and who am I to call DJjaffacake a liar?

), then the Mountain has been THREE different people and I never noticed. I don't know if that adds to my argument or if that means I don't pay attention... /edit
I'm assuming spoilers for the TV Show are cool here but if not, Season 1 and Season 4 minor spoils (no big plot things, just what the Mountain guy does).
In Season 1, he lops a horses head off. That's pretty freaking sweet...
In Season 4, he turns a man into a shish kabob and lifts him up in the air like a toy with a sword...
Hell, as actors, did either of them either speak outside of grunting noises?
Anyway, I'd go with Season 4. The dude is just stupid huge and that seems to be the only reason for the characters existence unless something major happens in the coming episodes.