If on the map they are 4 in apart, all you have to do is substitute it in the scale which is 1 in : 7 mi, so 1 in would turn to 4 by multiplying it by 4, so you have to multiply 7 mi by 4 as well, so it would be 4 in : 28 mi, so it would be 28 mi apart.
[Sorry for the bad explanation]