Part your after is LOSA3208 (standard) or LOSA3280 (heavy duty). They are discontinued. http://www.horizonhobby.com/SearchCm...chKeyword=xxxs
The xxx4 belt will work as long as you use the appropriate tensioner. This would be the same as using the "long black belt" that was an option part for the xxxs. I can't seem to find the kit on Horizon's website but can find it on Losi's - http://www.losi.com/Products/Feature...rodID=LOSA3277
I didn't think it would be discontinued as the XXX-4 is still made but going by the XXX-4 G+ explode view the tensioner is A3239 which is the one in the link above with the xxxs parts that is discontinued.
You maybe able to search the XXX-S Thread
and see what you come up with.