You use two (fronts or rears) of the four 4V outs as the
'home' RCAs aren't docked in the sled. I doubt that there's
any head unit that would have a problem with 4V outs fed to it.
In the worst case, just don't turn the empeg up all the way.

There probably are head units with 4 inputs but consider this:
The source material (songs) will only have 2 channels.

The 4 outs just give you the opportunity to apply different
amplification and equalizing to front and rears...