Impedance question

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.

There's no definitive answer other than experimentation. Usually, one tap will sound better than the other regardless of speaker. But for another amp, the tap can be speaker-dependent. If the amp is stable over reactive loads, then one tap should stand out over the other.

A measurement (in Ohms) of how much an electrical circuit pushes back when it's pushed.

The impedance value of a speaker is listed as a nominal value. No speaker has the same impedance across the audio spectrum. If the impedance vs frequency curve is published and accessable (the curves are for many speakers) you may want to take a look at the curve and see where and how much of the spectrum the impedance is at a given value. Some speakers nominally listed as 8 ohm dip to less than 3 ohms, for example. So there may not be as much difference as appears from the nominal value. That being said, other factors to consider are your listening habits and the type of music to which you listen. If you listen to highly compressed music ( most rock, pop, and hip-hop on cd) at high volumes you will pretty much need a lot of current to drive the speakers and your choice of speakers, for your system, narrows. If you listen at lower volumes and/or the genre is recorded with a wider dynamic range and is, by its nature of a wider range in volume, the sensitivity and impedance of the speakers is less important - while clipping may be a problem at the peak volume levels and result in distortion, you probably are not going to overheat the drivers because you won't be at the clipped level very often or for very long so at least frying the speakers won't be a problem. Speakers are by far the biggest single factor in the sound of a system, I would first find a few models that sound good when driven with a similar powered tube amplifier, without regard to the impedance values and sensitivity, and then bring those measurements to bear on your decision process. Eliminating 4 ohm speakers from consideration at the outset may unnecessarily limit your options.