For the purpose of Nav exams you are only likely to be required to define a position to one nautical mile i.e. 1 minute of longitude. Because we now have co-ordinates, originally designed for aligning an INS and now GPS equipment capable of giving positions to within a few yards, there will be more than 4 digits in the position. You simply don't need anything after the fourth digit, assuming its in degrees and minutes, any other format is useless for chart navigation.