From my understanding, as with a lot of planes, the gross weight limiting factor on the 4P with either the TSIO550 or the PropJet is the landing weight that can be handled by the gear.  I don’t know if the gear has been beefed up over the years or not though to go along with these increases.  I think there is plenty of power for takeoff at 4000, especially in the propjet, but if you bounced a landing in with 4k, you’d be in trouble.