Compatibility of 8 ohm amp with 4 ohm speakers?

If the amp has no 4 ohm rating, you run the risk of it overheating or being shut down by its protection circuitry, if any.

4 ohms will require more current (but less voltage) from the amp to deliver the same power into a 4 ohm load as into an 8 ohm load. Some amps just aren't up to it.

Typically I'd expect the marketing people to publish the amp's 4 ohm spec if it had a good one.