There are all kinds of video file formats. The computer video and now mobile phone video formats are growing. There are times when you will need to convert from one video file format to another.
If you can't find a converter in a commercial software package that will help you convert from MP4 to 3gg then you might want to visit VideoHelp.com Tools page.
It is not just video converters. There are encoders, decoders, DVD authoring software, video editing programs and more.
This website really is for intermediate/advanced computer users or those folks that have the patience to figure things out. There are tutorials and guides but some of them are out of date. That doesn't mean the software doesn't work but you will have to make adjustments depending on your current operating system and the version of the software.
Look at the screenshots, to see if you want to work with what is being presented.
You want to read the description of the tool and then visit the author's website. Is the author still there? Is there a support for the software?
And certainly before you download anything please have some sort of spyware/malware protection. There be pirates everywhere.