Sadly on a +4 there are (IIRC) two pulleys that the reverse of the belt runs over so any markings are usually long gone before the belt needs changing.
Doing an online search, I came across holley.com and their recommendation is:
with old belt removed and tensioner at rest, pass a length of string or wire round the pulleys in all the right directions, measure its length and subtract 16mm to determine the actual belt length required.
I also read somewhere else that if you cannot find the exact length, up to +/-3% is acceptable.
