Okay so I'm revising my thoughts on wiring then. Which I have no problems revising, I'd rather, as Darque stated - be on the safe side with this stuff.
- 4gauge from battery to the distro (step up fuse size from 50amp to 125amp)
- 8gauge from the distro to the external connector on the Thin Client
- 16gauge inside the Thin Client, from the connector to the M4.
I'll go for the connector fixer has listed.
About the relay that you were mentioning, the only place you might want to consider putting one is on your ignition line between your m4 and the ignition. I haven't looked much at the M4 but I assume that it has a remote line and that's what you indicated your using in your drawing? I would just look to see how much that remote line takes current wise. I kinda doubt that it would cause any major issues but better safe than sorry.
Hope this helps!