4" vs. 4.5" bolt pattern

OK I have a 70 Dart with 9in drums, (don't worry I'm replacing them with disks), but I don't know where to measure to determine if it is a 4" or 4.5" bolt pattern. Any advise on how to measure the bolts or at least where to measure to and from.

:snakeman:
If your replacing them?Just ask for 4.5 bolt pattern?73 and up were the large bolt pattern(LBP)