On 4s go w/ a 9xl....if you can spend another $30- or so , go w/ a Medusa...
http://www.rc-monster.com/brushlessmotors.php?pg=3 the 2200kv it is a much better motor..
1. MBX-6 T8 1900KV, RX8 ON 4S
2. MBX-5T 1520, MMM ON 5S
3. MBX-5 ONROAD CONVERSION 1515, MMM ON 5S
4. MRX-3 ON ROAD CONVERSION 1512, MMM ON 6S
5. TEN T 2650 T8, MMP ON 3S