Tvad has given you excellent advice. I would add that in the case of a 4 ohm speaker you have to be careful to note whether the sensitivity is specified relative to 1 watt input, or relative to 2.83 volts input (which is often the way sensitivity is specified, because 2.83V corresponds to 1W into 8 ohms).
If the spec is based on 2.83V, the sensitivity of the 4 ohm speaker is only 84dB for 1W input, which doubles the number of watts that are required to reach a given volume level, relative to what would be required with an 87dB/1W speaker.
Regards,
-- Al
If the spec is based on 2.83V, the sensitivity of the 4 ohm speaker is only 84dB for 1W input, which doubles the number of watts that are required to reach a given volume level, relative to what would be required with an 87dB/1W speaker.
Regards,
-- Al