I've used this one
avi to mp4 here a few times and it worked simple and easy for most files I tried.
it should be as simple as select your file, choose a destination path and file type and click convert.
I used it to make some avi's into mp4 to play on my phone and tablet.
the thing to watch for is, the simple programs that make it easy also limit your options.
when you compress or convert a file, it doesn't simply pack it into a smaller container taking up less space, it may also trim data from things like audio or video resolution. the 4 gigs in an avi file is probably 60% audio. and so it will narrow the frequency range.
meaning, if you play it on a laptop, pc, phone or tablet then you probably wont hear a difference, but if you play it through a home theatre system, you just might hear a huge difference.
something like this
avi converter is still simple to use, but will give you more options as far as format and bit rate to choose from that fits your needs better without killing the original file.