the 4 ohm rating

Did he try a tube clock radio? They do sound better you know. After you change the filter caps and the coupling caps. Custom wire and resistors are nice too.

Sorry for the long response:

Many speaker designers feel that speakers are 'voltage driven' and thus design speakers around that idea. This allows for a speaker to have dual woofers (in parallel for 4 ohms) and thus the amp will produce twice as much power so that the woofers will be driven hard enough to keep up with the high frequency units of the speaker. A good example of this sort of design is the B&W 802. The concept of 'voltage driven' speakers also allows for the amplifier to compensate for driver or cabinet resonance by responding to the resultant impedance bumps.

Now, not all amplifiers are capable of this 'voltage paradigm', but any amplifier that is will be seen to produce a constant voltage into any load. A good example of an amplifier of this sort might be a transistor amp that makes 100 watts into 8 ohms, but is 200 into 4 ohms. Sometimes it is interesting to point out that such amps will only make 50 watts into 16 ohms.

*So*, with the above two examples used together flat frequency response will be the likely result.

However, not all speakers comply with the 'voltage paradigm' nor do all amplifiers. Examples of speakers that are not on the voltage paradigm are horns, ESLs and planar magnetic speakers (ex.: Maggies). These types of speakers are on a 'power paradigm' where flat frequency response from the speaker is obtained by flat power response from the amp, regardless of the impedance of the load. This is often because the impedance of the speaker is not as closely related to built-in resonances. The high impedance of an ESL at low frequencies is a good example- this impedance has nothing to do with resonance in the speaker.

The kind of amplifiers that are on the power paradigm are: most tube amplifiers (tube amplifiers with very high feedback being the exception) and low or zero feedback transistor amplifiers.

This is a major reason why you have to be careful about matching amps to speakers.

Now back to your question about 4 ohm speakers: 4 ohm drivers are not any more likely to get damaged, and tube amplifiers for the most part are not likely to put any more power into a 4 ohm speaker then they would do into 8.

A couple of good reasons to think carefully about a 4 ohm speaker: speaker cables are a lot harder to build for 4 ohm setups. Also, most tube amplifiers will produce more power and lower distortion (regardless of design) when driving an 8 ohm speaker.

If you have a transistor amplifier there is no real reason to avoid 4 ohm speakers unless the amp is not really rated for the load. Yes, the amp is working harder but if properly designed this is not a problem for the amp, although it will produce more heat.

So if you plan to use tubes, you might want to think twice about 4 ohm speakers, but if using transistors it might be the other way around. Either way you want to be careful, not so much for the damage issue, which I think is a red herring, but more for the reason that your investment in the amplifier is best served with a speaker (all other things being equal) that is properly matched to it.