FAST? With the 4:1 your tank WILL be much slower than with a stock 3:1 gearbox. If you want slightly faster than stock yet with a little more torque look at the Taigen 3:1 gearbox with their 380 black can motors. The Taigen 3:1 gearbox ratio is slightly lower than Heng Long/Taigen/Torro "basic" gearboxes. The Taigen 380 motors have slightly more torque and higher RPM ratings. Running at the stock 7.2 volts the Taigen will give you a better combination of low speed handling yet a faster top end speed over a basic stock one. If you bump the volts up to 9.6 which the HL/Taigen/Torro MFUs can handle you'll see even better all around performance from either the stock or Taigen gearbox motor combinations especially if have or plan to upgrade to metal tracks.

In my HL M41 that is mentioned in this thread at the start I ended up going with the Taigen 3:1 with their black 390 motors (had a need for even more power do to heavy metal/rubber padded tracks) running at 9.6 volts. It has proven an excellent performance choice. Faster than stock yet with much better low speed handling.

If you choose to move up to 9.6 volt you will need to remove or modify your battery box to make space, something many of us do.