Almost any audio amplifier will work with 8 ohm speakers (with some exceptions such as fixed voltage amps.). However , as you lower the impedance of speakers you connect to a amp, you place more demand on it. Lower impedance means more current will flow through it's output devices (usually transistors). More current means more heat.

4 ohm loads present a more difficult load for amplifiers. The lower impedance means electricity will flow more easily. That means the current flow will be higher. Higher current means more heat will develop in the amplifier, especially in the output devices. Amps typically have themal protection circuits. If you your amp is shutting down due to thermal overload, you are driving it too hard, not giving it enough ventilation, or the amp is not working properly.

Amplifiers sold for pro audio applications, and for home audio would usually be able to handle 4 ohm loads with no issues,such as our FP series and DSP series and D series amplifier. Receivers may shut down under heavy load. Or perhaps their longevity could be reduced, because they were not really designed for 4 ohm loads. Lower impedance is not suggested, such as connecting two 4 ohm speakers in parallel.