8 ohms amp drives 4 ohms speakers

What will be the biggest problem when using 8 ohms amp (60 wpc) to drive 4/5/6 ohms speakers (about 86db)? Thanks a lot for your answer ^_^ ^_^ ^_^ ^_^ ^_^
I assume your amp is tube. Output transformer saturation can be a problem which will reduce your effective bandwidth, due to the high current demands a lower impedance will require. At lower volume levels, it should be fine.

But what exactly do you mean when you say your amp is 8 ohm?
If it's a transistor amp, it will likely run hot, trigger any protection circuitry at higher volumes, and perhaps even sustain damage in the worst case.

If it's a tube amp with an output transformer, consider that the output tranformer reflects the load impedance back to the tubes. In addition to Aball's comments, the output tubes will see too low a plate impedance and draw excessive current. Again this may potentially cause damage or blow fuses. However most output transformers have taps for common impedances.

I guessing that you're using a transistor amp without ratings below 8 ohms. If it has no 4 ohm rating there's probably a reason. Also 60 watts seems a bit light for speakers of such low sensitivity. I don't think you have a good match here.

Remember that the listed impedance of a speaker is a nominal values. Real speakers usually have large impedance fluctuations with frequency. A 4 ohm speaker may have dips to 2 ohms or less at certain frequencies and rises to 8 ohms or more at others.
lose of dynamic range or severe compression