Depending on what your plans are for the final output, h.264 may be a better option. Also, check out the available presets within export settings. As for .3gp vs .mp4 extension, look under the multiplexer tab.
The "MPEG4" format Adobe uses is for old cell phone video formats, which are pretty useless these days.
You want to switch "Format" to "H.264". This will export a .MP4 file encoded with H.264. There are lots of available presets, I would recommend starting with one of the YouTube ones specific to your video's resolution and frame rate.