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!