The difference is the resistance to the amplifier. If a subwoofer is 4 ohms on the amp, then it gets less wattage. Usually with 4 ohm to 2 ohm difference is that the ampflifier can put out twice as much power to the subwoofer because it's half the resistance. Dual 4 or dual 2 can only be hooked up to 2 or 1 ohms to an ampflifier unless you go up to 8 or 4 ohms on a subwoofer. More heat is generated, so be sure with a 2 or 1 ohm sub, you have a cooling fan built in the amplifier. And whatever you're hooking up, make sure the amp can handle it, with true wattage ratings and stability.