I've had a full set of Losi wheels on my car, I don't know if they where xx-4 or xxx-4 as I burrowed them from a mate. But I can report they fitted just fine.
Normally I use the black durga wheels, or the white 501x wheels. If you are looking for somewhere that stocks them, I get most of mine from here
http://www.brockmodels.com/, or from here
http://www.racingfactory.fi/