I bought couples of iTunes m4v movies and I hear avi is best for editing, is that right? How can I convert iTunes m4v to avi in a easy way? I am grateful for any help.
AVI and M4V are both simply containers, and neither (to my knowledge) are "best" for editing. What's best is to put the content into the container in such a way that HitFilm can use it smoothly, which is where codecs come into the conversation. Which operating system are you using? That will help determine the best codec options for your situation.
In general ProRes is the best format to transcode to for Mac, Cineform the best for Windows.
You may not convert iTunes m4v to avi directly by a common video converter, you need a DRM remover as M4V are usually refers to the iTunes M4V videos which are DRM protected. You might check this one DRmare M4V Converter to see if it can help you or not.