[Unsullied_Spy]

4s on a 5700 is pushing it, 5s or 6s is asking for trouble. I would go with a 4600 or 5700 depending on how hard you want to push it. I had a 5700 in my Rustler for about 2 years and it was very powerful and ran decent temps at 18/90 and 18/86 gearing. An FLM tranny case is a good investment. No more cracking at the hinge pins, runs smoother and quieter, and absorbs a ton of heat out of the motor (I've heard of people losing 20-30 degrees off their motor temps by going to the FLM case) and then put a clip-on heatsink on it with some thermal paste. That should be enough to get a 5700 running very reasonable temps in most weather.

EDIT: If you can afford it, get a Neu 1506 or 1509. I picked up my 1506/1Y off eBay for $105 shipped and it beats the pants off my 5700, Velineon, and the Hacker motor I had in there for a little while. It has the same fun, spunky driving characteristics of the 5700 but gobs more torque than the Velineon and runs cool no matter what I do to it. IIRC I have 13/56 gears in it (Jato spur with a 5mm bore 32p pinion) and it never gets more than luke warm on 2s lipos, no matter how long I go. With the FLM tranny case my MM would overheat before my motor would break 150 on 3s lipo and it was flipping over on it's lid at 50-60 MPH.