I just studied the drawings of the 12L3 and 12L4 on the Associated web site. If I picked up all the part numbers correctly, for less that $100 you can convert a L3 to L4. There isn't that many parts. That's even with the $45 chassis, new motor mount (which it doesn't look like is needed) and using list cost on the Associated web site. Does not include the axle, I imagine about everyone running competitively already has a big ring diff.