This webapp can be used to convert Libretro VBA format (.4gs, .srm) and .4gz files to sav, and sav to Libretro VBA format.
All of these file extensions are used to mark different formats for storing GBA game save data.
sav is the universal format. Libretro VBA format (.4gs, .srm) is a format used only by three specific emulators: the libretro fork of VBA-M, VBA-Next and 44VBA. The defining trait of a Libretro VBA format save file is that it always has a size of 139264 bytes (139.3 kB or 136 KB). More information here.
For Game Boy (Color) games, the format is exactly the same as the sav format; you can "convert" a Game Boy Libretro VBA format file to a sav just by renaming the file to change the extension.
Please keep in mind that although Libretro VBA-M and VBA-Next use the ".srm" file extension for their unique save file format, most ".srm" files are not Libretro VBA format. This tool can only convert Libretro VBA format saves.
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: