I use my 8 ohm receiver to power 4 ohm loads. It's never overheated, you just got to know what your doing. Power is power, and current is current. V = IR doesn't get any simpler. I = V/R. If your load is half of 8 ohms, you draw twice the current, thus twice the power at any particular Voltage, or Volume.
Since my receiver(Onkyo) goes between 0 - 70 in volume setting, I just make sure when running the 4 ohm load I don't push the volume beyond what the receiver can handle. Since those numbers are in dB, 60 vs 70 setting is 10 times less lound, or most likely 10 times less voltage. To be conservative, when running the 4 ohms load, I just make sure I don't go beyond 40. I'll make the assumption that Onkyo bloats the numbers and this receiver when operating under a 8 ohms load can safely provide current when the volume is set at 50-60. Switching to a 4 ohms load, I want to stay lear of that number so I keep my volume at no more than 40.
Usually when playing movies, I have the volume setting at 40 for the 8 ohms load. When driving the 4 ohms load for listening to music, I usually have it at 37, or 3 dB 1/2 volume down.