In very basic terms, valve amplifiers differ from solid-state designs in that the output valves have an impedance characteristic unsuitable for driving speakers directly, so transformers are used to turn this into an impedance suitable for the task.
Unfortunately, due to the nature of this conversion, an impedance mismatch between speaker and amplifier can alter the power output a little, and create more distortion and limitations in the frequency response. As a result, some valve amplifiers offer a choice of 'taps' - literally connections from different windings on the transformer - to suit speakers of different impedance.
If your speakers are 4ohm, then you should use the 4ohm tap. Using the 8 ohm one won't damage the amp or speakers, but may not give as good a sound.
Just to clarify further, impedance is here a measurement of the load the speaker puts on the amplifier - ie how much it impedes the flow of energy. So while lower impedance may seem ideal, and can allow some power amplifiers to double their output into a 4ohm load compared to that into 8ohms, for example, it isn't always so. Too low an impedance and energy may flow out of the amplifier faster than the amp's power supply can replace it, thus causing distortion as the amp struggles. This can damage speakers.
A final point is that speakers don't have a fixed impedance: the figure given is a nominal one, sometimes accompanied by a minimum figure, as impedance varies with frequency. The degree to which this happens depends on the speaker design: some speakers have more stable impedance/frequency characteristics, while others can have quite notable variations.
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