I agree with the other posters, TT bumpers are not engineered with withstand the kind of force 4 bikes will have. I like your thought of additional bracing, but the bumper itself is thin wall. I would suggest a dual hitch receiver and mount the bikes from the truck hitch in front of the TT.
Ok! I didn’t think of that. Do you think it would be an issue with having those bikes and the trailer hooked up at the same time?
I don't think so but you'd have to measure. The dual hitch receiver will add about a foot to the distance, but the rack could hang back far enough to limit a tight turn. It might also cover your hitch ball release. You might also consider a tailgate pad. I've used those. The front forks/wheels hang on the tailgate. Less obtrusive, although it eats up bed space.
