Of course the 5th channel will only have one RCA in!!!! One channel = one signal = one RCA!!
All monoblock means is that the whole amp only has one channel to avoid channel interaction. This amp simply has 5 channels - one RCA per channel.
The 5th channel is however of class-D design, a lower frequency switching design so it can't reproduce high frequencies like the usual class A/B designs. It is however more efficient in its operating range but restricted to subwoofer freq's. All channels are 'separate' as such. There are 5 amps in a 5 channel amp.
Just thought I'd clear that up!
Oh, you would most definately have to run 4G or even 2G for that amp over the distance from your battery to the boot. If the amp won't take 2G just put a distribution block in the boot! The amp has a max. current draw of 80A so a pretty reasonable fuse will be needed too.