4ohm and 8ohm speakers?

Many speaker manufacturers could care less about the difficulty of load they present to an amplifier. Perhaps most famous in this area of companies still in business are Jim Thiel and Gayle Martin Sanders. There is nothing wrong with this philosophy. The design of low or high impedence is not harder or easier, just different. Impedence is not the only factor in the ease at which a loudspeaker is driven. Sensitivity and phase angle must also be considered. Crossover design is perhaps the most disregarded part of the equation. The higher the order the crossover, the more difficult it will be to drive. First order is easiest, 4th order much more difficult. The impedence of a speaker has nothing to do with its sound. 4 ohms will not sound "better" than 8 per se, and vice versa. Many drivers are even available in either impedence. It is the choice of the designer as to which is picked. Also whether multiple drivers are used in a loudspeaker is a determining factor. Wiring two drivers in parallel halves the impedence. Wiring them in series doubles the impedence. Current is what moves drivers, and big drivers(woofers) specifically. By an amplifier driving 4(or less ohms), it will be demanded to deliver more current. Many amplifiers have no trouble doing this. Big, solid state amps are terrific at this. They simply grab hold of a woofer, and are in charge. The bass produced is often phenomenal. You should check it out. I feel this type of stuff has peaked(mid 80s - mid 90s). Look at the ads back then for power amplifiers(Adcom, B & K, Belles, Bryston, Carver, Classe, Counterpoint, Krell, Mark Levnison, NAD, Rotel, Threshold, YBA, etc.). They will often claim the ability to drive 4, 2, or 1 ohm loads. Some even said they were able to drive a dead short(I NEVER tried this at home). You could say this was the golden age of solid state. Over time, some of us got tired of the horsepower war. We began to focus not on sound quantity, but rather quality. Many of us rediscovered tube amplification. Tubes do not produce the current solid state is capable of, thus they do not work well into low impedence loudspeakers. As we moved to tubes, there was an increasing demand for more easy to drive(higher sensitivity/impedence) loudspeakers. Companies like Coincident, Soliloquy, and Meadowlark emegerged to fill this void. Today, there is all manner of approaches. Solid state, tube, low impedence, high impedence. It is a good thing, really. Some people like vanilla, some like chocolate. We now have the choice when we go to the store, many flavors are available.

I read somewhere that a 4ohm version of a driver will have a greater maximum output. Does anyone know if this is true? This might be a factor.

The issue of 4ohm versus 8ohm is too simplistic. For example, my valve amps really rock with some (but not all) of the 4ohm Thiels, and drive Martin Logans beautifully, but don't do much with the 8ohm Silverline Sonatina. If you just look at impedence and sensitivity then the Thiel 1.5 should be as hard to drive as the Thiel 2.3 or even the 3.6 - but try it and you will find the 1.5 is actually very easy to drive.