The opening of cylinder drain cocks is generally arranged to take place simultaneously on all cylinders by operating a single control in the cab, and the effect should be four horizontal emissions of steam at the leading end of the engine with every revolution of the driving wheels of a 2-cylinder engine, six with a 3-cylinder simple, and so on. If in certain cases you have noted an emission at every fourth exhaust beat only, there seems little doubt you have mistaken for a drain cock effect the effect of a loose gland.
Source: wiktionary