Here is a pretty good wire gauge calculator.

It depends on the servo current and the voltage. For instance, if the stall current is 2A and you are running on a standard 5v then you could work it out in the calculator;

Power (Watts) = Current x Voltage, so 2 x 5 = 10 watts (check your servo current it probably isn't 2A) Enter the distance you want, 4 feet in your case Leave loss at 10% unless you know different Hit calculate

In this example it would need 24AWG wire.

Although 24AWG works out in theory it does seem a bit thin in application. Since the wires on servos are usually in the 20-22AWG range I would just stick with those gauges if possible. One rough test that I use sometimes is just testing the temperature of the wire with your hand while the servo (or other application is running under load), if it ever feels hot it's probably time to move to a lower gauge (thicker sized) wire.

When buying wire you always want to use a step or two thicker gauge than required. There are three reasons. First is you don't want wire size to be your current bottle neck. Second is that even wire has resistance and the thicker gauge the wire the less resistance you are imposing on your electric power source. Third is heat , if you figure wire to be just within your requirements you may find the wire getting hot at times. Because many wire companies do not have a name brand reputation to uphold they are usually sold under a thicker gauge and higher rating than they can handle. They usually are thinner wire by a size or two and the individual strands are thick instead of thin with a higher strand count. Overall diameter and strand count are your two most important ratings of a wire size. So when buying wire it never hurts to oversell yourself on the better wire for efficiency , safety and overall best performance from motors and servos.

Excellent guys. Thank you. I think I've wrapped my head around it :D

Daniel