Speaker's amplifer minimum power recommedation

Speaker manufactures often list in their specifications a minimum amplifier power recommendation.  Is that power recommendation into a standard nominal 8 ohm impedance, or is it a recommendation based on the speaker's published nominal impedance?  I am looking at a speaker with a 4 ohm nominal impedance and the specifications recommend a minimum power of 40 watts.  Is that 40 watts into an 8 or 4 ohm impedance?