Did we ever determine whether it was ok to add ATF+4 to a tranny that probably has ATF+3 already in it?

And, is the list of ATF+4 brands just as good as the Mopar brand?

And, what about additives like Trans-X?

OK....lots more reading tells me that using the +4 with the +3 already there is good. Dunno about the quality of other-than-Mopar +4.