I agree with these guys but If you want an answer with a bit more elaboration...

The speaker ratings you mention are the relative or average impedance of the speakers. Impedance, as the word suggests, is a form of resistance -- to "impede" or hinder. Put simply, Impedance is a "frequency dependant" resistance. As frequency changes the measure of resistance changes too. Unless an audio speaker is only going to reproduce a single frequency, such as a test tone, the resistance of the speaker will change with the varied frequencies of the musical material: But it is possible to give speakers an average impedance rating. So as you asked, "what's the difference between 4 and 8 ohm speakers?" the 4 ohm speaker offers less relative resistance than the 8 ohm speaker does. What practical value does this information provide for you? It may seem backward but the speaker with a lower impedance rating is "relatively" harder to drive by and amp or receiver than the speaker with a higher impedance rating. Why? In order to understand that, you have to understand what the speaker is impeding. Electrically, the speaker is resisting the flow of electrical current. The higher the rating, the lesser amount of current draw on the amp's power supply: The lower the rating, the greater amount of current draw on the amp's power supply. You can think of it in terms of water systems if that helps... a small diameter pipe has more resistance (in the form of friction and reduced capacity) than a larger diameter pipe. But because the large diameter pipe has less resistance to flow and greater capacity, it must be supplied by a larger reservior. In other words it places a greater demand on its source. Hopefully that make sense to you, but if not, let me know and I'll try to explain it differently.

As a practical matter, if you're using these speakers in a 2 channel stereo setup and don't intend to listen at very loud levels, most decent quality receivers will drive them adequately; especially if you use them with a sub. Since they are only rated down to 100 hz, a sub is going to be a good idea anyway.

The brand names that have been mentioned in the other posts tend to offer good current capacity into lower impedance loads so there is certainly nothing wrong with those recommendations. But as I said, if you're using these for "stereo" listening at light to moderate volume levels, many mass market receiver brands will provide all the power that you need. If you are thinking about using a set of five for surround sound, then that would probably NOT be a good idea.

One other thing to watch for is an impedance setting switch which is sometimes available on receivers. The thing you need to now about this switch is that it is a protective device only. It limits the current draw on an amp to protect receiver and/or speakers but does not do anything to improve the sound of lower impedance speakers -- in fact, it can negatively effect sound. You are much better off finding a receiver which post good "watts per channel" numbers into lower impedance loads than buying a receiver simply because it has this switch.

Like I said, there was nothing wrong with the previous recommendations in the other posts. But the way you phrased your question led me to believe you might be interested in more of an explanation of what these ratings are all about and how it effects receiver choices. I hope I didn't make it more confusing for you.

Q