He could not bid two hearts, which would promise six cards.

Or bid three hearts and risk missing a spade fit?

That would have had no chance, but West naturally bid four hearts.

South was content to bid two hearts, which ended the bidding.

With favorable vulnerability, one would expect West to bid at least three hearts.

If East had bid two hearts, what would have happened?

He opened one spade, and South was not strong enough, in the modern style, to bid two hearts.

East had to bid two hearts to allow for the actual situation.

What would have happened if somebody had bid five hearts?

He should either pass throughout or bid two hearts on the first round.