Back in the days on no holds barred competitions with no weight classes I saw Renzo Gracie defeat former UFC Heavyweight Champion and kickboxer Maurice Smith in a match in Japan with a figure four arm bar.
I was a fan of this arm bar after seeing that and here is how I learned to do it. I have to ask Renzo for his tip on how to get it from the Americana side.
Here is how to catch the figure four arm bar from side control.