The term 'VERY FAMILIAR WITH' can be expressed by the crossword solution UPON, which is a four-letter word. UPON is an adverb that means 'in a position of being on top of or over something; at or near the upper surface or upper part of something; directly preceding in time or order; immediately after.' It is often used to express a sense of familiarity with a certain topic or situation. For example, one might say 'I am very familiar with this topic, as I have been studying it for years UPON years.'