So here's a dumb electrical question

Downsides: more points of failure, a single point failure might could overload the remainder and cause a ripple effect failure, more work.

Upsides: none, unless the smaller wire is very cheap.

Before doing it, I'd scour wrecking yards for trucks and vans and yank the longest cables you can find. Likely a similar cost as 4x 12ga (any that's actually load rated) and so long as you're not picky about wire color you should have plenty of pickings from vehicles already missing their engines.