sportracer: You should be able to call HPI USA Customer Service and get an updated gear set with the one-way brace at (949) 753-1099. They have update kits that include 2 sets of the new gears, the new 0.1mm copper shims and the purple one-way brace. Definitely call them up and explain youíre a Pro 4 owner that has the old version gears.
Aku-Man: Hopefully my PM covered everything. Let me know if it didnít.
Aurra Sing: The Pro 4 is definitely capable on Carpet. I would look at our Optional 3mm Chassis
and also the Optional 2.5mm Upper Deck
upper deck for a full time carpet ride. Together they retail for $97, but should be much lower than that at most shops (maybe around $80).
brushless: Check your one-way also. If your diff screw was broken, the front wheels would still move (you mentioned no drive at all). If not, then check your input shafts and pins. As other have suggested, the gear mesh may be too loose alsoÖ which would definitely produce a clicking noise if the teeth are skipping over each other.
Trinity2: If you got your car in the US (kit#185), give HPI USA customer service a call about the new gears and the one-way brace.