If you don't get a satisfactory/economically-sane answer on actually obtaining FM9 licenses (which I suspect is possible through Adobe Volume Licensing) ...
Presumably the other 2-4 writers are on newer versions of FM.
If they are on FM10, you're covered, because they can save as FM9. They need to avoid new features added at FM10.
If they are on FM11 or 12::
- Designate the FM9 user to be the final handler of all files.
- Have the FM1x users again avoid using new features added after FM9.
- Have the FM1x users save their final work as MIF to .book.mif and .mif.
- Have the designated handler open the MIFs and re-save as FM9 binary .book and .fm (be sure to re-open and test some of the resulting documents on FM9 to make sure the FM1x people didn't inadvertently invoke something from the future).
If the other writers are on older versions of FM, also no work-flow problem. FM9 can open their work, but those authors obviously can't use any FM9 features, which might be a document content problem, in which case upgrade them to FM12 and use the MIF route.