IF you are picking speakers that are properly matched to tube amps in the first place, those having relatively flat impedence curves with no really large sharp peaks or dips in the impedence values, the the issue of 4 or 8 ohms is not a big deal and the amp wouldn't 'work harder for either'. The output for 87db speakers would be the same with either 4 or 8 ohms. You just pick the impendence connection that sounds best to you.
However if all you are taking into consideration is the nominal impedence value, not the actual curve or the minimum impedence, stick with 8 ohm speakers. A speaker with a nominal 4 ohm impedence could have a substantial dip down into the 2's which would make it hard for many tube amps to drive, absent a 2 ohm tap.
These issues have been discussed extensively in this forums. Perhaps a bit of archive reading will help you understand them better.
Hope that helps a bit.
However if all you are taking into consideration is the nominal impedence value, not the actual curve or the minimum impedence, stick with 8 ohm speakers. A speaker with a nominal 4 ohm impedence could have a substantial dip down into the 2's which would make it hard for many tube amps to drive, absent a 2 ohm tap.
These issues have been discussed extensively in this forums. Perhaps a bit of archive reading will help you understand them better.
Hope that helps a bit.