Convert AVI to MP4 using FFmpeg with and without re-encoding the AVI file. Also, learn to install FFmpeg and do a lossless conversion from AVI to MP4 using FFmpeg.
Ok, if you are in a hurry to convert AVI to MP4 using FFmpeg, then simply run the following FFmpeg command. It should work for most of the use-cases.

ffmpeg -i input_filename.avi -c:v copy -c:a copy -y output_filename.mp4