A Y-splitter does not divide the voltage. Simple electrical laws of parallel wiring; current doubles, voltage stays the same. If you have 4V preamp outputs, then both amps will receive 4V (or very close thereto). The increased current needs may cause a slight drop in signal strength....but we're talking no more than a couple tenths of a volt.
An example of this would be your battery. Everytime you wire something new to your battery, it doesn't divide it's voltage between all of the connected circuits does it? I sure hope not...because if so that new amplifier would only be receiving less than 1V of power input //content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif But no, luckily for us it does not. All the components still receive the full 12V, up until the point that the current draw gets so large that it begins decreasing the voltage. But even then all of the components still receive the same amount of voltage...it is never split between them.