No, WINFAP 5 does not support WINFAP 4 project files (.feh4) nor the old Peak Flow Datasets (version 9 and earlier). WINFAP 5 uses the new NRFA Peak Flow catchment descriptor xml files, and has improved data auditing functionality.
To allow continued use for existing studies, the legacy WINFAP 4 can be installed alongside WINFAP 5 and we currently issue WINFAP 4 serial numbers with each WINFAP 5 purchase. Both versions are available from our downloads page.