Don't use the primary key for actual meaningful data. Create a calculated field or auto-entered field to generate your topic number. By removing the topic field from the primary key it should also be easier to handle the year component.
You may also want to have the number as a text field rather than numeric. the "004/15" may cause issues if FileMaker tries to resolve that as a calculation.
Thank you for your input.. I was thinking about that route, but I was kind of hoping that some FM mojo could make me use one less field in that table.
I will give that a whirl..