I often get asked about this and some speaker manufacturers like to avoid stating average impedance when they quote speaker sensitivity…
In 2002, Paul Messenger described beautifully the paradox of 4 Ohm speakers.
‘Over the years, speaker makers have tended to evolve their designs to suit the typical transistor amplifier which is essentially a voltage source with a low source impedance.
Halve the speaker’s impedance from 8 to 4 Ohms and the speaker draws twice the current and power from the amplifier. This extra power gives an increase in apparent (though mythical) sensitivity. Once one manufacturer adopts the strategy, others feel obliged to follow.’
A 4 ohm speaker actually draws 2W when provided the voltage required to provide 1W into an 8 ohm speaker so a 93db 4 ohm speaker is no easier to drive than a 90db 8 ohm one!