Honestly 700rms would be ok on ofc 4 gauge, but if you have to buy wire, you might as well get 0 gauge. I had a re audio xtx 3000. Putting out 600 @4 ohms and 4 gauge worked perfectly, but i also only had a 3ft run because my battery is in the rear.
I already have the 6gauge hooked up to my kenwood amp. I will be purchasing 0 gauge but I'm just wondering if I can get away with using the 6gauge for the time being with a 4ohm load until I get new subs to wire down to 1 ohm and do 1500-2000wrms.