Is there a reason that you must use iMovie?
What are you using as your source to play on the big screens?
Assuming that you are using a laptop.
Why don't you install VLC player for the Mac OS, Burn the mp4 files to a Dvd as data. Use VLC player to play the mp4 files for ya. No conversion necessary. You could also use a USB stick.
Assuming a Dvd player.
Does the Dvd player support dvix and avi files or is it a standard dvd player?
Another software program to consider