It is not just the focal length. Coverage is important. If you intend using the lens for 5x4 make sure it has enough coverage for the large format and any movements you may wish to make with the camera. Example: I have a 105mm Solinar that does a good job on 6x9. It will illuminate the 5x4 frame, but the edges are too soft to be usable.

A good place to start with this is http://www.largeformatphotography.info/