While I certainly agree that the 4 hour mark (I can only remember one flight in 4 years at the airport that ever waited that long for departure away from the gate) is a good point for the decision makers (the Captain, dispatch and station ops) to have a pow-wow on whether or not it's time to call it a day, I would be fearful of setting a hard and fast wall at which the only option is to dump the flight. Someone needs to be able to look at the big picture. If you're at the 4 hour mark, but flights are rapidly being released for departure, it's probably best to wait it out and blast off, or you're never gonna get home.
I'm sure there is some wiggle room with this, or at least I hope so. I'm sure AA wouldn't tie themselves down by writing this into the Contract of Carriage or anything like that.