Many on the board have made their own fan using a computer fan, me included. There are a few threads on the board that talk about it and provide instructions on the build process. For me, I have not made jerky, so I can’t say about that, but it has helped on low temp smokes like salmon and sausage.
Here’s one thread on it, there are a few others as well.
https://www.smokinitforums.com/index.php?topic=6828.0
That said, I have a 3D so a bit smaller than the 4. You could find a computer fan a bit bigger to pull the air if needed.