It matches the output impedance to the speaker's impedance. In theory, such a match affords better performance and power transfer.
You may want to check your speaker specs and select the tap close to the minimal value of the speaker impedance curve. Impedance is generally spec'd at a nominal value but the impedance of all speakers varies with frequency. For example, I use the 4 ohm taps to drive nominally 8 ohm speakers that dip to a little over 3 ohms. .
FWIW, You've chosen the right tap when you get the sound you enjoy most. Since you have speakers with a nominal 4ohm impedence I'd start with the 4 ohm tap. If your speakers don't have a serious dip in their impedence in the bass you will probably get the best match off the 4 ohm tap. But, if the speaker does have a serious dip in the bass your sound may become enimic and may benefit from using the 2 ohm tap.
Anyway, its likely that there is not perfect fit in any evnt, so just choose what sounds best to you. You won't damage anything.
what does it do when you try different taps thanks
I agree with the preceding responses, but to elaborate a bit further:
The different taps represent different points on the secondary winding (the output side) of the output transformer. For a given signal condition, the 8 ohm tap provides the greatest voltage; the 4 ohm tap provides a smaller voltage; and the 2 ohm tap provides a still smaller voltage.
I believe that a typical design will provide at the 4 ohm tap approximately 0.707 times the voltage of the 8 ohm tap, and at the 2 ohm tap approximately 0.707 times the voltage of the 4 ohm tap. The figure 0.707 is 1 divided by the square root of 2.
The result of designing the taps that way is that exactly the same power (voltage x current) will be delivered into an 8 ohm load as into a 4 ohm load as into a 2 ohm load, assuming the load is purely resistive and independent of frequency.
An additional result, which is important, is that the plate circuit of the output tubes will then see a load impedance (equal to the speaker impedance times the square of the turns ratio of the output transformer) which is the same regardless of whether an 8 ohm, 4 ohm or 2 ohm speaker is connected. That allows the voltage and current operating points of the output tubes to remain at or near what the designer chose as optimal, regardless of speaker load.
But along the lines of what the others have said, speaker impedances are not purely resistive and can vary considerably with frequency. So I agree with Newbee that it would be best to start with the 4 ohm taps, which match the speaker's rated nominal impedance, and evaluate the possibility of using 2 ohms based on what you hear. But keep in mind that the 2 ohm tap will output less voltage, and therefore less power at frequencies for which the speaker's impedance is higher.
al i did try 4 ohm taps the amp was gettig to close to the end were i like to play my music my amp is solid state it seam to have the same out put but useing the 2omh taps does two things lowered my out put meters and is more detailed in the sound i think my maggies dip down more then i thaught thanks
Almarg, your post is correct regarding tube amps, however Majic's amp is solid state. McIntosh uses autoformers on their top solid state amps, so it is a different ballgame.
I found out that experimenting is the best option with a Mac Mc402, for example, Gallo Nucleus 3.1 speakers (8 ohms spec) sound better connected to the 4 ohm tap.