Sabum or Sabumnim
(Hangul - Sabum 사범 - Sabumnim 사범님)
Sabum is Korean for Master, as used in Hapkido. In general, rank of 4th dan (4th = sa in Korean) is called Sabum.
The suffix -nim is added to show a measure of respect.
When you address a teacher, you will therefore use the term Sabumnim. The teacher will call themselves sa bum (you never use the suffix -nim when you're talking about yourself)