amplifier that can output more power than your subwoofer can handle, so your gain should be set to limit the amplifier output to less than 500w.
music. depending on how alpine rates that pre-amp (RCA) output, that 4v rating is either it's 'peak' output or 'RMS' output. if it's an RMS rating, then it will only output 4v when listening to a constant tone at full volume. if it's a peak rating, then it will only output 4v during musical 'peaks' at full volume. either way it will never output 4v at normal listening levels.