There have been quite a few questions lately regarding the safety (or otherwise) of driving 4 ohm speakers from amplifiers only rated to drive 8 ohm loads.
Rather than just add to the mass of unsubstantiated conflicting opinions (e.g. "amplifier XXX is built like a tank, so it must be able to drive 4 ohm loads"), I thought I'd attempt to apply a bit of science to the subject. Hopefully this will make people aware of some of the issues involved in driving low impedances.
In relation to ensuring reliable long-term operation, three of the most important parameters for a transistor are it's voltage, current, and power ratings. Exceed any of these three at your peril.
Voltage : In any half decent amplifier design the output transistors will have a sufficient voltage rating to withstand all "normal" operating conditions. Only abnormal events such as lightening strikes are likely to cause failures.
Current : The transistors need to be able to handle the maximum currents taken by the load. Speaker impedance can (and does) vary considerably with frequency, and often dips well below the nominal 4 or 8 ohm value. Amplifier designers are well aware of this fact and counter it either by using higher rated transistors with large peak current capability (good) and/or by incorporating current limiting circuitry (not quite so good, but better than blown outputs!). In practice a good amplifier design will withstand the ultimate over-current event - the accidental short circuit of it's outputs (but don't blame me if yours doesn't!).
Power : Every transistor has a maximum power dissipation rating. This is not a fixed figure but varies with the temperature of the transistor. For example, a transistor might be rated to dissipate 100 watts at 25 degrees centigrade but only 20 watts at 100 degrees centigrade. The amplifier designer should provide cooling, in the form of heatsinks, to ensure that the maximum temperatures are not exceeded.
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