The main reason manufacturers use 4 volt pre-outs is to reduce input noise thereby giving a cleaner signal into the power amp. With the higher pre-out voltage, there is less chance of ignition noise, stray RF, etc. modulating the input signal when it feeds into the power amp. This is especially important if the amp is mounted far from the HU (like in the trunk).
You should be able to use just about any power amp with your HU as long as it has an adjustable gain control for the input. My current amp was not designed specifically for a 4V input, but works just fine with my Nak CD40z when I dial down the input gain.
-Wes
98 SE Sport
