Both .gp and .gp4 files are projects from different versions Guitar Pro, which means that gp to gp4 conversion is typically done for the purpose of re-saving earlier file types to more modern format. Today, both of these files are obsolete and replaced by GPX format, so if you still need files for version 4, you might have to obtain that particular version of the software. Or you can try to reach out to the Guitar Pro support or online community.
Updated: July 25, 2023