As a VERY happy customer of the M2 tactical with Comfortech stock and rifle sights, I'd strongly suggest going that route over the M4 anything - because it's an M4EvilScaryBlackShotgunOMGOMGOMG-RUN!
Seriously, if I had it to do all over again, I'd do EXACTLY what George suggested, and get the M2 Comfortech with a 21" VR. Realistically isn't going to be any harder to maneuver than a civilian legal 18" gun, and the vent rib would allow Aimpoints new RDS very nicely. Doesn't look "scary" - but still plenty easy to use for serious use. In the free world, you can run a LONG magazine tube (I'm partial to Nordic) with, IIRC 10-11 rounds in the tube. Going into occupied territory? Put the factory mag cap back on and learn to reload faster!
Otherwise, I'd look hardest at a lever gun of some sort - I'm partial to "Trapper" length pistol caliber versions. I mourn the days of the relatively affordable, easy to find Winchester 94 big loop trapper - had one in .45 Colt and .44 Magnum - and stupidly let them both go. The 30-30 is a better choice if you want to press it into hunting service, but is, IMHO, needlessly powerful (with lower capacity) for (Mobile)Home Defense.
I wouldn't bother with anything AR based or otherwise even vaguely military-derived, ESPECIALLY if you're planning on traveling into California.