Would this work?

ok if you had a single voice coil sub that was 4 ohms could you add a 4ohm resister inline in the positive power cable from the amp to present the amp with a 2 ohm load to give the sub more power from the amp?

no.

ok just out of couriousity. why?

well, you actually can ..... and it will lower the load by 1/2. Problem is, the speaker and resistor will each dissipate 1/2 of the total power. So if you had a 100 watt amp (at 4 ohms) connected to speaker and a 4 ohm resistor - the amp would put out 200 watts. The speaker would receive 100 watts and the resistor would receive 100 watts, so you would be right back where you started from, only now you're amp is drawing twice as much power from your electrical system. There actually is a way to do what you want to do, problem is it would be cheaper to just buy a 200 watt amp.