What's with 4 ohm speakers?

I don't know the answer to the OP's question but I do wonder if it has something to do with cost. Take a 4 ohm and an 8 ohm version of the same driver unit: Is it less expensive to manufacture the 4 ohm version?

Four ohm versions are usually more sensitive. A good amplifier properly mated to a 4 ohm speaker will usually deliver more output than it would into an 8 ohm load. With more output, you can play louder with less distortion and more headroom.

Well, I guess I should have checked the thread again before posting because Atmasphere practically wrote just the opposite and I trust his knowledge about such things far more than mine. He didn't answer the OP's question, though, so somewhere along the value chain, I still wonder if it has something to do with cost.