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.