The short answer is that you can convert them. Whether you can simply use the converted file immediately, or need to test and make a few adjustments here and there... Can't really tell you. You'd have to test your converted files and see.
One free experiment you can try is to download a trial version of FMP 10 and see what happens when you attempt the conversion.
Thanks folks - Gee, I feel lame not having found my own old post/question on this. (i was
wondering where that was; obviously we haven't moved too swiftly on this task ; )
Any event, thanks for your help and we'll give it a try per your suggestions.
Filemaker 5.5 can run on OSX, as well as ver 6.
Each shares the same file structure of ver 4, so the move would be relatively painless.
The problem is finding those versions of Filemaker. It is one of the most difficult of programs to find legal earlier versions.
Rather than expending the money to make a minor update, I would look to upgrade to a more current version, 8.5 at least.