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.

Just because its power rating was given at 8 ohms does not mean that it cannot push a 4 ohm load. Of bigger concern is that the speakers may be 4 ohm nominal, but may dip down lower at some frequencies. Many planar speakers do drop down to 2 ohms or less, and require large amounts of current. OTOH, If its a steady 4 ohms and the minimum power requirement is well within the rated capacity of the amp at 8 ohms, you may be OK. Older Nikko may not be able to deliver enough current, tho, to make the Maggies work to their capability.

Thanks,good informative answer.

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