You can operate 4-ohm speakers with an 8-ohm amplifier if you use caution and understand how impedance works. Most modern amps can be used with speakers that have a total load at or above the minimum impedance rating. For instance, a 4-ohm amplifier can run any total load of 4 ohms or above, including 16-ohm cabinets and speakers. In order to calculate your total impedance, you must know the impedance of the speakers or cabinets and how they are connected. Parallel wiring lowers the impedance. Series wiring raises it. Assess your connections carefully before using your amplifier.
Plug in a speaker cable from your 8-ohm amplifier to your first 4-ohm cabinet. You will need at least two 4-ohm cabinets to perform this task because a single 4-ohm cabinet will overheat an 8-ohm amplifier. Ensure you have plugged into the amplifier's speaker output jack and not a line out or other nonamplified output. Those jacks will not power a speaker or cabinet.
Plug another speaker cable from the 4-ohm cabinet into another 4-ohm cabinet. This connection will give you a total impedance of 8 ohms for the amplifier. When you run in series, from one cabinet to another, you add the impedance of each cabinet to determine your total impedance. In this case, it is 4 ohms plus 4 ohms, or a total of 8 ohms.
Wire two 4-ohm speakers in the same cabinet in series. This method will raise the impedance to 8 ohms. Since you are dealing with individual speakers, you must wire them in series to get the desired results. Wire the speakers as per the instructions. Plug your single 8-ohm cabinet into your 8-ohm amplifier. This procedure will allow for the most amount of watts going to the speaker cabinet.
Use an impedance matching device. Plug a speaker wire from your amplifier into the impedance matching device. From the device, plug another speaker wire into the 4-ohm cabinet. This method will match the impedance load to the minimum impedance that the amplifier can handle. In this case, it will change the impedance recognized by the amp to 8 ohms from the cabinet. These devices are expensive, but can reduce the risk of failure due to overheating.
Use thicker speaker cables the lower your impedance goes to avoid heat damage.
Never run an amplifier below its minimum impedance. Doing so can cause overheating and excessive heat can cause permanent damage to electronic components.