Eight-ohm speakers can be run with a 4-ohm amp. One 8-ohm speaker plays loudly with only half the current from the amp, but if two 8-ohm speakers are connected in parallel, the resistance in each speaker falls to 4 ohms to match the amp.
According to Ohm's law, R = V/I, where R is the resistance or impedance in ohms, V is the potential difference in volts and I is the current in amperes. When impedance decreases and the potential difference remains the same, the current increases. The increased flow of current can cause the amp to overheat. Apart from potentially damaging the amp, overheating consumes some of the current that is meant for the speaker, leading to less speaker performance. Thus, although the setup of 8-ohm speakers connected to a 4-ohm amp can work, it is not ideal. Ideally, a 4-ohm amp should be matched with 4-ohm speakers.