Breeze has you covered here. It's important to know the amplifier impedance and not go dangerously low. Most amps are 8 ohm to each channel. using your present speakers you have 8 + 4 = 12 divided by 2 = 6 which is less than nominal. you might be able to get away with it but the more power i.e., loudness the impedance will drop. I would match all speakers to the amp impedance and be sure to wire them in parallel, that is each positive and each negative connected together from one speaker to the other.