My opponent's cav model charged my Warrior. I promptly surrounded his cav model with three more Warriors. He won anyway. He said, "All four Warriors are knocked down, because the cav retains all bonuses." I replied, "But the cav only charged one model...and retains the knockdown bonus for that one model...but doesn't add the knockdown bonus to include the uncharged models...it isn't realistic...knock down comes from being charged." I gave in, as he is an honest and more experienced player...but I'm still not wholly convinced.

What would they rule in the GT?

Your opponent is right. The odds might have been on your side by adding warriors, but it's riskier if you fail. And it is "realistic", at least as realistic as an abstract representation of reality could be. Consider the turn as a snapshot in time...the horse hasn't actually stopped moving and still has all its inertia and kinetic energy through the turn.

That is why when trying to stop a charging hero you have to consider whether to sacrifice one dude (and risk a heroic move) or go for broke and surround them stopping the heroic but potentially losing 3 or 4 men.

Great; thanks for the clarification.

Still doesn't quite hit a realistic note...three attacks after the initial charge, and all four on their backs...but that's rules. Good to know.