gp4 to gpx conversion is typically searched by users that own old version of Guitar Pro project (in this case version 4) and want to save it in the latest GPX format or at least import .gp4 files to later versions of Guitar Pro. This should be possible and Guitar Pro actually supports all older formats, so if you struggle with this somehow, it would be perhaps best to reach out to their support or online community.
Updated: August 24, 2023