Convert GP3 to GP4

Unfortunately, Guitar Pro formats lacks backward compatibility so it is not possible to produce .gp4 file using the latest version of the program. Your only option would be to find version 4 of the program and import .gp3 file to it so you can save it as .gp4 file. Without the required version of the program, this conversion is most likely impossible.

Simply put, it is impossible to convert GP3 to GP4 directly.
Thus, there is no so-called gp3 to gp4 converter or a free online gp3 to gp4 converter to be found.