Impedance question

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.

Its OK to use 4 ohm speakers as long as you use the 4 ohm taps on your amp. But in my opinion you should be looking at speakers with a little higher sensitivity, something like 90db 1watt@1meter or higher. Every 3db up is almost like doubling amp power.

The purpose of the audio output transformer, which has taps for 4, 8, and sometimes 16 ohms, is to make all these loads look the same to the output tubes.

Generally speaking, most 4 ohm speakers are less sensitive than most 8 ohm speakers. Since tube amps don't double wattage when halving impedance, you will be hard pressed to find 4 ohm speakers with decent efficiency. As stated by Newbee, look for at least 90db.
Final note, 4 ohm speakers can be run off of 8 ohm taps usually without any adverse effects to the amp. Use which ever taps sound best.

Newbee makes a good point, due to the high output impedance of most tube amps, speakers with wildly fluctuating impedance curves will cause modifications to the frequency response due to the voltage divider effect, and are to be avoided, unless you like the sound of the modified response. I would also point out that the four ohm tap has approximately twice the damping factor, or half the output impedance, of the eight ohm tap on any tube amp and so will control the bass response differently.