FWIW the 4 ohm tap is there so you can load the tubes correctly. The tubes are likely wanting to see about 3000 to 5000 ohms from plate to plate and your speaker is much lower.
The transformer does exactly what it says it does. It transforms impedance, voltage and current.
If you load the 8 ohm tap with a 4 ohm load, the load that the tubes will see will be a lot lower. The result will be higher distortion and a loss of power. You may heat the tubes up a little more too, as the loss of power I just mentioned is actually being dissipated in the power tubes rather than your speaker.
Some output transformers don't do 4 ohms so well, and by using more of the windings (8 ohms) sometimes you can get better response. Usually this tactic is limited; the better transformers will operate correctly on 4 ohms, IOW the better the amp the less using the 8 ohm tap will help.
So most of the above responses have some truth to them but I thought if you were interested, now you have the whole scoop.
