No I haven't and I would not even consider it. Fact is it is not legal in most states. Tying on 4000 lbs behind a 5er would jeopardize the safety of the person doing it as well as the safety of any others in the area.
Not sure where you are located but that is considered triple towing. Cannot be done legally from Florida to Maine but I have seen rigs doing it. Not sure when they might be stopped but an accident would certainly put you in great jeopardy. i think it's legal in most states west of the Mississippi but California most likely will stop you to see if you have the proper credentials.