mpg4 to 3gp conversion is probably just a variant of mp4 to 3gp conversion, considering that MP4 is the most commonly used file extension for MPEG-4 container.
Modern multimedia converters or editors should recognize MPEG-4 container regardless of the actual file extension, but you can always rename the file to restore the default associations for programs installed on your Windows computer.
Updated: October 5, 2023