The higher, the better. The higher the pre out voltage, the stronger and cleaner the signal going to the amplifier is. While results in not having to turn th gain up as much, and the amplifier doesn't work as hard, usually increasing its lifespan. Look at some Eclipse decks like my CD 8445, it has 8v preouts.
It doesn't double the power, it increases the signal.
It gives you a better S/N ratio.
And as said, it allows you to keep the amp gains turned down to keep the amp running cooler and with less distortion.
If you plan to use an amp, then I highly suggest 4v. The 4v units will only be around 2.5v at the highest unclipped volume setting; which is generally about 75-80% of full volume for most HUs. (the voltage output is logarithmic)