I am no expert but from a pure electronics standpoint hooking an 8 ohm load to a 4 ohm source is OK. In order to get maximum power transfer both the source and the load should be equal but if you increase the load, speakers in your case, resistance you reduce the current flow but effectively dissipate more wattage at the load = more efficient. Efficiency is of little value to your listening experience but it should mean less heat generated in the amplifier output stages which is always a good thing. In otherwords, no problems. Don't confuse efficiency with maximum power, I know someone is going to jump on my case for this.
