If you put one stud at 12 o'clock, and then measure from outside to inside of the two studs just below, that will give you the bolt circle. An easier way would be to buy a 5 buck plastic piece that has from 4" to 5 1/2", most tire and wheel suppliers have. I hope this helps.
The 4" is the bolt circle, so you would have to measure strait across to an invisable circle line between the two lug nuts to get a perfect 4" center too center measurement.
if you drew a 4" diameter circle and held it up, the outside edge would hit the center of every stud. That is how it is properly measured. You can measure by skipping one stud but it is not an exact measurement... the 5 on 4" means 5 holes on a 4" circle....