What - you've never seen E used to express voltage or (ElectroMotive Force)?
Bizarre!!
You can acheive 4 ohms by wiring two sets of 8 ohm speakers in parallel, getting roughly double the volume.
Actually, strictly speaking, this is quite correct - all things being equal!
(although it's easy to see why people would think it isn't)
True, a 4 ohm speaker will not give twice the volume of an 8ohm speaker - as was said, this is primarily decided by the speaker's sensitivity.
However, if you place one 8ohm speaker on an amp ouput, you'll get a given volume (for arguments sake, say 90dB).
If you then place another identical speaker in parallel, it will have exactly the same voltage across it and will draw exactly the same current as the first one, and so will also produce the same volume, 90dB. The equivalent impedance seen by the amp will halve (which if the speakers were exactly 8ohm, would now appear as 4ohms total)
Now, 90dB + 90dB = 93dB
This 3dB increase is a doubling of the acoustic output (volume?)
However, we don't hear in a linear fashion.
It takes a 10dB increase for us to hear an apparent doubling in loudness (also referred to as volume - the terminology is poor, but that's not exactly a new issue).
That's why I added "strictly speaking" and "all things being equal"
It also assumes a decent amplifier, which won't clip when you halve the load impedance, and will stay linear (ie halve the load impedance, double the output current)