You can spend a moderate amount more money on a comparable quality 4 ga kit and have "room" left over to add a sub amp in the future. 8 ga kits are good for around 500W. A true 4 AWG OFC amp kit can handle another 1,000W above that. A CCA 4 ga kit will handle less power, actually less current, but it would also suffice for a moderate addition in amplifier power in the future.
Basically the cheapest kits will use CCA wire that isn't true to the gauge advertised, e.g. cheap 4 ga wire will often have equivalent metal content to 8 AWG "true" spec wire. The most expensive kits have full OFC wire, which means they handle higher current loads and there is enough metal in the wire to meet the AWG spec, which in car audio is spec'd by CEA.