pro's and con's of a 4 ohm speaker?

Cwaz - Power will be inversely proportional to load (assuming good power supply) while perceived loudness goes in (1/3.5) power of the amplifier output power ratios.

In you case: Loudness = (8/4)^(1/3.5)= 1.22 (22% louder with 4 ohm speaker)

8 ohm speaker will also double your damping factor (damping of the speaker).

Al, I always forget about tube amps since the last one I had was EL34 100W guitar amp I built about 40 years ago.

As I understand it combined plate impedance gets divided by speaker impedance to get transformer turn ratio. If plate impedance is 1k then transformer ratio for 4 ohm speaker will be 250. There might be negative feedback involved but it will lower impedance in the same ratio thus 4 ohm tab impedance will be always half of 8 ohm tab impedance.

Is there any benefit of using lower impedance speakers with tube amps? I remember 16 ohm speakers and heard of 32 ohm designs. Didn't low impedance in speakers come with SS amps?