I am building a 10 X 14 shed , the rafters will run the 10ft span .How high will i have to go with ridge board for a 4/12 pitch ?

A 4/12 pitch means that there is a 4" rise in a 12" run. That is 4" up for every 1 foot horizontal. Since the ridge board is in the center (I presume) of the 10 feet width, that is 5 feet from the outer wall to the ridge board. So 5 x 4" is 20" to the top of the ridge board.