I need the update for 4th gen. I own a small business and my customers want the new update

You will have to give more information. What do you mean 4th Generation. iMovie is now 9.0.8 requiring Lion 10.7.4 or later. You customer will have to get their own updates from Apple, presuming they own iMovie.

Oops. My bad. Got my forums confused. You are looking for iMovie for iOS, the latest version being version 1.4.