Hindi rin naman ako expert, kaya meron din akong disclaimer na "for me" sa umpisa.
The receiver's 4 Ohm setting is preferable when the receiver gets hot (not just warm) at high volume use. As a safety precaution, the 4 Ohm setting on the receiver steps down the current to prevent overheating.
The side-effect is that a reduction of the amp's current capability would reduce the dynamic range of the output. This dynamic range limitation would be most noticeable in the bass output. Medyo hihina ang bass.
Using the 8-ohm amp setting, well-built receivers such as Yamaha can still handle even 4 ohm speakers. But the risk of overheating the amp is still there. My personal view is that you should not take that risk.