Use ffmpeg. Don’t buy any tool that shows up in the Google search results. They are all rip offs.
It is a simple command to convert AVI to mp4.
ffmpeg -i input.avi ouput.mp4
ffmpeg -i input.avi -c:v libx264 -crf 19 -preset slow -c:a aac -b:a 192k -ac 2 output.mp4
There are numerous applications like
All of them show up in the first of the google results but unfortunately most of them are just rip offs. Do not download the trial version if you do not have any intention to buy the paid version. Even after paying there is a high chance, you don’t what you want.
Try the tool that everybody in the whole planet uses, ffmpeg. It is a very advanced application. With just a bit of google with “how to convert AVI to MP4 using ffmpeg” can get you necessary help.