Is there a data dictionary out there for Baan 4c that would give me the tables/fields I need to know that are tied to an accounts payable system, mainly the Invoice Header, Invoice Line Item, Payment, and Purchase Order tables?

For all tables in Baan it's easy to retrieve the table definition, the most essential thing to know is the relations between the tables. A part of this is stored in the table definitions, another part is stored in the business logic ( sources ), sources are normally not available in Baan.

If you to create management information the information standard available should be enough to to the job, if you want to interface information I would prefer to use AFS as this quarantees your business logic will be used to store the information in Baan IV without creating data discrepances.