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.
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.