What 3GP to MP4 actually writes
Clips from older phones are 3GP and will not open in current editors.
The 3GP is re-encoded to MP4. The picture stays as small as the phone recording was.
Quality and the 3GP file you should keep
Do not expect HD. There is nothing in the 3GP to upscale into real detail.
A related step, if you still need one, is MP4 to 3GP.
When 3GP to MP4 is the right page
3GP to MP4 sits on this site because 3GP and MP4 are not interchangeable, even when they look similar in a file list. Clips from older phones are 3GP and will not open in current editors. Very old handsets asked for 3GP. Current phones do not. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4.
The 3GP is re-encoded to MP4. The picture stays as small as the phone recording was.
What you give up by leaving 3GP
3GP is a small video format from older mobile phones. Quality is limited by that source. Creating 3GP uses H.263 and drops a lot of detail. Do it only for a device that still says 3GP.
Keep any better copy you have. 3GP is a last resort.
What you gain, and do not gain, with MP4
The download has to be MP4, because that is the format the next step named. Phones, browsers, Instagram, YouTube, and WhatsApp all expect MP4. If the other person can open only one video format, MP4 is that format.
A new encode cannot look cleaner than the MP4 you started with. A remux on a desktop is the lossless path when the streams are already compatible.
A test that matches the real file
Start from the question the other software asked. If it asked for MP4, 3GP to MP4 is the matching page. If it asked for something else, switch tools instead of hoping a similar extension will pass. Clips from older phones are 3GP and will not open in current editors. The 3GP is re-encoded to MP4. The picture stays as small as the phone recording was. The limit is the part people skip, and it is the part that decides whether the download is usable. Do not expect HD. There is nothing in the 3GP to upscale into real detail. The file stays on your device. The first run downloads a converter, so keep this tab open. A phone runs out of memory on long files. A computer is the safer place for a big clip. Check three things in the download: it opens, the 3GP material you cared about is still there, and the file size or the layout is what the form described. A new extension alone is not those three things. If a portal, a teacher, or a client rejects the file, send them the original 3GP as well when you are allowed to. 3GP to MP4 is a convenience copy. It is not a new master. Video files hide the real problem inside the container. 3GP to MP4 re-encodes on your device. It does not silently copy every audio language and every subtitle the way a desktop remux can. If you needed those extra tracks, this download is the wrong result even when the picture plays. Convert a one-minute piece before a full movie, and stay on the tab while the converter downloads the first time.
Here is the decision for 3GP to MP4. You start with 3GP and you stop when a real program accepts MP4. An old 3GP can become an MP4 that editors will open. Soft subtitles such as ASS or PGS, and extra audio languages, are awkward or dropped when a tool re-encodes. If those two sentences describe a tradeoff you do not want, do not convert. Renaming the extension by hand will not do this job. Running the tool twice on its own output will not do it either.
Do not expect HD. There is nothing in the 3GP to upscale into real detail.
A related step, if you still need one, is MP4 to 3GP.
Other jobs start from the homepage, where each tool is named for the conversion it actually does.