Don, 4" short is norm but not always, sir! Could be 3, 4,5, even 5, depends on the limb design and such.
Make up a long tillering string with a timberhitch or bowyers knot and use it to find your brace.
Sounds like your going to build a bow, so you will need a tillering string. I would build it out of a high performance material and heavy strands, D-97, say 16 strands. Fiberglass is very abrasive!
Look foward to pics of the bow!