No, that's a floor, not a cap. If you are denied boarding and arrive at your destination more than 4 hours after you were originally scheduled to, the airline owes you at least 4 times the fare paid, up to a maximum of $1350. In other words, the floor has a cap. Even if you paid $1000 for your fare, the airline still owes you at least $1350, since 4 times the fare exceeds $1350. However they can, at least under the current rules, be glad they don't owe you at least $4000. They are, of course, allowed to pay you more, but they generally don't.