With a 4s battery I would suggest using a 20t pinion. That may seem odd, but that setup will yield ~40mph, which might still cause the motor to be undergeared.
SH Z-Car, Custom Crawler, 8s Savage, 12s XTM XLB 1/7 buggy, 4wd 4-link rear/IFS Pro4 truck, Custom Hyper 10 Short Course, Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz