I didn't think it was as simple as that?
I understood that it was down to the impedance i.e. the resistance of the speakers.
8ohm speakers offer higher resistance than 4ohm speakers, so "pull" less current from the amp. 4ohm speakers will "pull" more current and can potentially damage a weaker amp.
I am no tech expert though!
The impedance is (mostly) just the electrical behaviour of the speaker. What matters most is the acoustic behaviour of the speaker. The sensitivity number you see quoted is a simplified (and marketing tweaked ) statement of that. I'd guess that my home hi-fi speakers are around 88dB sensitivity. One of the biggest loudspeakers we make at work is 100dB sensitivity. That means with 1W input you get that many dB SPL out. If you double the power input you get another 3dB out, so to make the hi-fi speaker reach 100dB you'll need 2^4=16W into it.
Very few small hi-fi speakers can handle 16W in the lows without becoming somewhat non-linear (various distortion mechanisms). My 50W hi-fi amp can probably push this pair of speakers with 5" woofers to about 103dB at best. Meanwhile stick a pair of pro-audio speakers with 2x12" woofers on that same amp and they'll hit 103dB with 2W and the amp will run out of power at 115dB, comfortably twice as loud (10dB = doubling of loudness).
The joy of very very big hi-fi speakers is that they need so little power to drive them that you can vastly reduce lots of the distortion mechanisms inherent to sound reproduction - if speakers are equally well designed and equally expensive then the bigger they are, the more real they tend to sound.