You can import old project files to Guitar Pro and save it in the latest version available in your version of Guitar Pro. To produce version 4 projects (.gp4) you will need that particular version of Guitar Pro and no other, because the latest versions of the program do not support saving to previously used formats (it is not backwards compitable).
Convert Guitar Pro project to Guitar Pro 4 project.
We found one software available in the database for this conversion.
Microsoft Windows software - convert gtp to gp4 on Windows
Multi-track editor and music composer for guitars
Convert from gtp file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.