[Buddha-l] Mantric letters

[Alex Wilding]

There is a set of 4 syllables (occasionally there is a fifth) that turns up
in a number of mantras - can anyone tell me the origins of these letters: ha
ri ni sa
They are often associated with 4 dakinis or 4 elements. I guess they are the
initial letters of some set of things (substances? animals? qualities?) -
quite possibly it's extremely obvious to those of you who know. But not to
me - grateful for any clarification!
Alex W