Beer has a pH of about 4 when fresh, but this can drop to 3.5 or below if the beer is exposed to oxygen such that it sours, as is inevitable in a cask after dispense. Fresh ciders may have a pH as low as 3.3 and, when oxidized, even below 3.
Acidity is generally reported in terms of lactic acid, though other organic and inorganic acids are involved. The acidity figure should be reasonably constant, though slight fluctuations can be expected. Various strains of yeast, as well as lack of proper wort aeration, can affect acidity.
Abnormally high acidity can be an indication of bacterial infection of the wort and/or beer.