There is only one file type that uses the .WP4 extension:
WordPerfect 4 Document
Word processing document created by WordPerfect 4.x for DOS, the first version to include automatic paragraph numbering; overtook WordStar as the most popular word processor for the DOS platform.
WP4 files can be opened by Corel WordPerfect 4 or later.
Updated: August 14, 2010