I'd be better of with an 8-ohm speaker. They can dip down to 3 ohms or lower at certain frequencies. Even their DC or voice coil resistance can be in the 3.2 ohm range. In Solid State gear, when in doubt, always go for higher impedance. Even if your gear can accept 4 ohms, to be safe, I'd get a speaker set with 8 ohm nominal impedance. Lesser heat dissipation as well, as your amp is less taxed to send more current at lower loads. Just my thoughts.