As far as I know the only difference is in signal strength into the amplifier. The 4 volt preamp output would be best but the 2 volt will work as well.
short answer: a 4 volt preout will give a stronger, cleaner signal at the amplifier, with a higher signal resolution. end result is your amp won't have to boost a weak signal via the input sensitivity adjustment to get the signal in the target range for the input stage of the amplifier.