practicing the brahma viharas in meditation

Is there clear instruction about how one should practice the brahma viharas in the teachings? 1. Should one go through each one in order [metta - karuna - mudita - upekkha] or is it possible to do i.e. only karuna (in a formal meditation sitting) 2. Why is it that (from my perspective) many meditation teacher only seem to focus on metta ('loving kindness') meditation (and not the other 3). Is there a particular benefit or is it important to practice it in order to get into the 'cycle' of the brahma viharas?