I'd have the giants commence battle against those puny creatures.
I tend to assume that most opponents don't get their bearings in the first round of combat. That, plus everything more or less happening simultaneously, means that if they don't immediately surrender or run, they tend to fight to the end of the first round--even if that kills everyone.
With a smaller group of opponents like those four giants, I'd say that any survivors at the end of round 1 would be able to realize they need to surrender or run. I might even let some of them realize it in the middle of the round, though depending on temperament, they might need a few seconds (ie, the rest of the round) just to cool off long enough to realize that surrender or flight is a better option that death.
When you are dealing with, say, a dozen bandits or goblins or such...unless half of their numbers die in round 1, they won't even know who's winning until the end of round 2, what with so much happening all at once, lack of visibility, etc.
So summary: Opponents make initial assessment and fight or not (for these giants = fight), small groups get a chance to reconsider by the end of round 1 (during the round if I'm feeling generous towards them), while large groups rarely get a chance to stop and assess the situation until the end of round 2.