Yes, twin #4s in parallel would be like single #1 or whatever it comes to for ampacity.
The two DC terminals for pos and the two for neg can take #4 on the Vector, so if you choose single fatter wire, now you have a problem. The answer is to use those poke-in terminals you can put on the wire ends, which fit in the smaller set-screw holes that the wire itself is too fat for.
The Renogy drawing of its DC terminals is unclear. Might be over and under with set-screws above and below so it can take the two wires each side? Hard to tell. Anyway, the OP will soon know, then he can plan better.
Can Tire has a popular 3000w inverter that is often on sale, that also uses four 3 ft #4 wires.