There are a couple of ways to accomplish this:
1) You can use a self-joining relationship based on a field that holds the prefix (in this case, the year) and use the Max function. Be careful with this one; it's vulnerable to creating duplicates if two users execute at exactly the same time, so be sure to validate the sequential number as unique and re-generate if you get a validation error.
2) You can use FileMaker's intrinsic serial ID creation capabilities and just reset at the beginning of the year in your opening script, checking the year against the next value using GetNextSerialValue and then setting it, if necessary, using the Set Next Serial Value script step. The only thing you have to be careful of here is making sure you keep the serial number synchronized between copies of the database (if, for example, you have a development version and a production version).
Thanks as always Mike. Let me digest the options, give them a shot and I'll come back to you if I have any further questions. Much appreciated.