Hello Dan,
Using impoly in interactive mode does not allow for you to limit the user to a certain number of datapoints. If you know that you want an aligned rectangle, you could use imrect, but I'm guessing that's not what you're looking for.
You could make your own function relatively simply using ginput, which does allow you to put a limit on the number of clicks. Or you call it in a loop and draw lines between the points in between each click.
-Cam
