What's with 4 ohm speakers?

Whats' with 4 ohm speaker's? Its the damn drivers. Many of the tweeter's on the market from RAAL, ScanSpeak and other's have a 4 ohm impedance.

In tweeters the mass of the voice coil becomes an issue so I do not doubt that they would use 4 ohms over 8. All things being equal that would cut the voice coil weight in half.

I was incorrect on the RAAL ribbon's. They are all 8ohm impedance. ScanSpeak has 36 different tweeter model's in production. 23 of those models have 4ohm impedance. Vifa currently has 18 tweeter model's that have 4ohm impedance.

Ribbons are often 8 ohms as the ribbon itself is the equivalent of a voice coil.

Speaking of tweeters, Morel offers nearly identical MDT29's in both 4 and 8 ohm versions. As rated, the 4 ohm is 92 dB sensitivity and the 8 ohm is 89 dB. For a MTM, the 4 ohm would be more likely.

So, just by halving impedance and not doubling driver area and motors, you get +3 dB.