Ahh XB4 same as what I have, best bet is to use the 22-4 Front and Rear wheels.
First page of the XB4 thread has the wheels and hexes listed, also the XB4 thread will be helpful if you have more XB4 questions since others can help answer as well.
from first page:
Most who have gone to the 12mm hexes are using the +0.75mm (#365355) on the front and +1.5mm (#365356) on the rear using standard 12mm rear wheels and TLR 22-4 front wheels which have the same offset as the Xray 12mm front wheels (#369912). Car becomes very stable with this combination.
Also you really don't want to go any wider in the hexes listed, cause you'll run out of thread engagement for the wheel and nut especially with front wheel.