practicing the brahma viharas in meditation Is there clear instruction about how one should practice the brahma viharas in the teachings? 


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)
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?
Thanissaro Bhikkhu said the other Brahma viharas are applications of Metta, and that is why Metta is the core focus in the practice.


  Of these four emotions, goodwill (metta) is the most fundamental. It's
  the wish for true happiness, a wish you can direct to yourself or to
  others. Goodwill was the underlying motivation that led the Buddha to
  search for awakening and to teach the path to awakening to others
  after he had found it.
  
  The next two emotions in the list are essentially applications of
  goodwill. Compassion (karuna) is what goodwill feels when it
  encounters suffering: It wants the suffering to stop. Empathetic joy
  (mudita) is what goodwill feels when it encounters happiness: It wants
  the happiness to continue. Equanimity (upekkha) is a different
  emotion, in that it acts as an aid to and a check on the other three.
  When you encounter suffering that you can't stop no matter how hard
  you try, you need equanimity to avoid creating additional suffering
  and to channel your energies to areas where you can be of help. In
  this way, equanimity isn't cold hearted or indifferent. It simply
  makes your goodwill more focused and effective.


http://www.accesstoinsight.org/lib/authors/thanissaro/headandheart.html