This webapp can be used to convert 4gz and 4gs/srm files to sav, and sav to 4gs/srm.
All of these file extensions are used to mark different formats for storing GBA game save data.
sav is the universal format. 4gs/srm is a format used only by three specific emulators: the libretro fork of VBA-M, VBA-Next and 44VBA. More information here.
For Game Boy (Color) games, the srm format is exactly the same as the sav format; you can "convert" a Game Boy srm to a sav just by renaming the file to change the extension.
If this tool doesn't work, Please copy the debug output and paste it into a new issue on the issues page, along with your as-detailed-as-possible description of the issue. I put a lot of effort into making this, and I don't want that effort to go to waste.
Debug output: