Everything starts with the ready to fly weight. A 4 lb plane x 100 watts per pound = 400 watts required. Then you decide what voltage battery you will use, probably 3 or 4 cell I would guess. Take the desired watts and divide by the battery voltage to get amps required. So 400 divided by 11.1 volts (3 cell) = 36 amps. So you need a motor that can handle at least 36 amps (3 cell) and an ESC that can handle it as well, be sure to add a bit of a margin say 10%. Finally consider the anticipated speed of the aircraft, fast planes like higher KV motors and higher voltage batteries than slow planes because more volts and more KV mean more RPM. Lower volts and KV mean lower RPM but usually more torque for bigger props which are nicer on slower planes.
Therefore I would guess you should consider a motor of around 800 KV or so for 4 cell batteries and about 1000 kv for 3 cell batteries. You will NEED a wattmeter to find the prop that draws the amps and watts you calculated in step one. Then go fly and tweak the prop selection to get the performance you want. Check watts and amps with each prop change to be sure you don't exceed the limits of your motor, esc or battery.
Your questions might be more suitable for the electric guys on the proper forum. However we like to be helpful here on the 1/2a glow forum.
The electric guys are probably more knowledgable than this 049flyer!
Last edited by 049flyer; 11-18-2013 at 12:36 PM.