tatyana,
If you are talking about using tube amps, your circumstance might be a bit more problematic than using a speaker with a higher nominal impedance. Low impedance speakers tend to be harder to drive and are more prone to having problems with frequency response anomalies when driven by amps with higher output impedances, such as many tube amps.
That is not to say that your ProAc will not work well with tube amps, you just have to be more careful and accept that some amps will not work well. I have heard other ProAc speakers, such as the D48, with tube amps and the particular combinations I heard sounded very good.
The 4 ohm rating of your speaker should be no problem with any decent solid state amp, and given the particular model you have, I am assuming something far better than decent would be used.