depends on the quality of the receiver and also the signal being applied, if you are *pushing* the speakers, trying to get things quite loud for a long time say for example a house party, I would in the least make sure the receiver has good ventilation and check how warm it is periodically... You can also check the specs of the receiver to see if it can take it 
running @4 ohms will not necessarily wreck the amp, it will just produce more heat because at 4 ohms, giving everything else being constant; you will have twice the power being output, so if a 'scene' averaged 20 watts at 8 ohms, it will average 40 watts at 4 ohms.
The other thing is I *BELIEVE* that a 4 ohm speaker relative to an 8 ohm speaker (everything else being equal) will be 3 db louder (+3db= 2x the watts) so you could just turn down the volume proportionally and 'convert' your receiver being at -3db to "mean" that you are really at 0 db
running @4 ohms will not necessarily wreck the amp, it will just produce more heat because at 4 ohms, giving everything else being constant; you will have twice the power being output, so if a 'scene' averaged 20 watts at 8 ohms, it will average 40 watts at 4 ohms.
The other thing is I *BELIEVE* that a 4 ohm speaker relative to an 8 ohm speaker (everything else being equal) will be 3 db louder (+3db= 2x the watts) so you could just turn down the volume proportionally and 'convert' your receiver being at -3db to "mean" that you are really at 0 db
