in my expierence with the rc18s I would stay away from that high kv motor, I did like the 5400 kv Castle motor with 3s and a 60/14 gearing the Castle motors do run a little hot. IMO if I did it again I would go with the long can Hyperion yc-22 or a Neu 1105/3y would be a screamer with 4s lipo and decent with 3s here is a link to the Neu motor

http://www.atlantahobby.com/shopexd.asp?id=6564

I have thought about using a seperate BEC if I tried 4s but I lost interest and my rc18 is gathering dust