I'd say it will break down like this:
Game 1. Carter 1st quarter, Vinny 2nd, Henson the second half.
Game 2. Vinny 1st quarter, Carter 2nd, Romo second half.
Game 3. Starter first quarter, back-up 2nd, Henson second half.
Game 4. Since it was so close to game 3, this one will be mostly Henson and Romo.
Parcells said in any eariler PC that he plans on treating the games like camp, he did not want to use 4 QBs in games.
It's important for Henson to play as much as possible in preseason, and if Romo does not show enough in camp, he may not play a lot in preseason. Which could help him clear waivers and be on the PS.