The volts from the head unit and the power source (alternator/battery) are two different things. Volts is only a measure of power, but not how you use it. For the power source, the more volts available, the more the amp can process into power for speakers. To answer your question, there's no reliable way to figure out how much an amp will put out based on knowing the voltage, because your car fluctuates in voltage, all amps are different, and manufacturers will make them different sometimes for features like limiting power so you won't fry the amp.
The voltage for head units IS more power, but not for the amp, just a cleaner signal. Depending on the voltage, it may seem like you're getting more power to your amp, because the sound signal to the amp will be larger and therefore your amp will play slightly louder. Primarily, though, the 4 volts is for a really clean sound signal, because the ratio of sound to noise is better.
RCAs work with two wires, in a basic generalization, one RCA carries the sound signal (which includes ambient and power "noise"), the other, the ground, carries the "ambient noise" or general power of the car. The amp then takes the signal, and then compares it to the grounded signal, and will be able to "remove" the ambient noise from the signal wire based on what it gets from the grounded/ambient noise cable. It's sorta like getting a letter from a friend with lots of extra words and sentences in it, then getting another letter that tells you what words to take out, so you can get an understandable secret message. Major overgenaralization and improper term usage, but that's baasically how it works. Therefore, the 4 volts will give you WAY more sound signal over/versus the ambient noise, and a better, cleaner sound.
To answer your more general question, the voltage from your car will fluctuate, so you can't really derive much from that. Your head unit's voltage, however will be steady, so when it says a certain voltage, it will stay that number. 4 volts will always be (should be) 4 volts.
Good luck,
Jon
[Edited by JTang on 12-18-2000 at 10:13 AM]