Even if you did this, the sub would still be a 4 ohm sub. It would represent a 2 ohm load to the amp, but the resistor would be absorbing (and heating up with) the extra power. No amp is perfectly efficient either so if you did this, the sub would more than likely not be getting as much power. Think of an amp that does 400 watts at 4 ohms. That amp will more than likely do about 600 watts @ 2 ohms. What you're doing there is taking a sub that will be getting 400 watts rms, wiring it in parallel with a 4 ohm resistor, and then feeding it 300 watts rms. If you add another 4 ohm sub, at least you'll be doubling the surface area of the woofers so it'll be louder.