the 4 ohm rating

Here is what one speaker designer has to say about designing 4 ohm speakers
"Thiel speakers have long suffered a reputation for being power hungry. Jim Thiel admitted that most of his speakers sport 86-87dB sensitivities and are 4-ohm loads. He could easily transform them into 8-ohm loads, but he prefers to avoid the concomitant loss of sensitivity by 3dB. He also uses 4-ohm loading since most well-designed solid-state amplifiers will double their output power into such loads. Why purchase this amp capability only to leave it unused?"

taken from http://www.soundstage.com/yfiles/yfiles200107.htm

Even if your amp can drive a more demanding load it will still work a bit harder doing so,as compared to an easier one.Think 4 cylinder vs 6 cylinder engines.The fact is that the internal circuitry will still run cooler on an easier load,and common sense dictates that there has to be a sonic advantage to doing so.

sirspeedy, even a Class A biased amp? My amp seems to run hot no matter what's connected to it. I think in my case it's just a matter of whether the energy is being used for musical purposes, or dissapated as heat.
Using your car analogy, my car (amp) runs pedal to the metal all the time. It's just a matter of if the wheels touch the ground and transport me somewhere, or simply races the engine, if the transmission is placed in park while maitaining with the pedal to the floor.
I can understand your reference regarding a Class A/B designed amp, I don't know that this is the case with a Class A biased amp.

To use Jc's analogy above, class A runs pedal to the metal all the time (i.e., it's class A). It won't get cooler or hotter unless the ambient temperature plays tricks...

Samadhi Acoustics offers an excellent full range speaker (28hz up) rated at 94db and 4 ohms. A custom interstage transformer coupled SET 300b with 7 watts can drive these speakers with ease. The transformers are custom wound Electraprint and designed for 4 ohm loads.

There is an excellent article on 4 ohm and 8 ohm speakers on Roger Russell's McIntosh history website. Here is what it says:

Impedance of Home Systems is Changed To 4 Ohms

All systems manufactured prior to 1993 are 8 ohms and have an industry standard sensitivity rating of 1 watt into 8 ohms at a distance of 1 meter. All systems made after this date are 4 ohms and have a sensitivity rating of 2.83 volts into 4 ohms. This turns out to be 2 watts into 4 ohms, making them appear to have greater sensitivity. To convert to a 1-watt level, subtract 3 dB from the rated sensitivity. For example: the LS350 is a 4-ohm system rated at 89dB for 2.83 volts. This converts to the industry standard rating of 86dB for 1 watt at a distance of 1 meter.

Generally, a 4 ohm speaker will play louder on a direct coupled amplifier than an 8 ohm speaker simply because it draws twice the power with the lower impedance. A few companies have used this strategy for many years. The idea is that when a simple switch arrangement is used in a dealer store to compare different speakers, the louder one is supposed to be the one that gets the sales—and this does help.

Reducing the speaker impedance to 4 ohms does not improve the accuracy of the speaker, reduce distortion or make any other improvements. However, it does create problems for the customer. It means that the customer must purchase heavier speaker wire to maintain the same low losses compared to the earlier 8-ohm speakers. The total DC resistance of the wire must now be less than 0.4 ohms instead of 0.8 ohms. It also requires better connectors at the speaker and amplifier terminals to make the contact resistance even lower.