The reason 4x5 appears to suffer less from diffraction for a given aperture is simply that the sensor is bigger, so that the blurring has relatively less effect.

Here's a quote from the forum discussion linked below:

> Yes, diffraction does depend on the absolute diameter of the aperture, at least that is what determines the angle of the diffracted rays. But with a longer focal length, the rays diverge farther before they hit the film, so the Airy disc (the fuzzy blob created by diffraction) becomes larger with longer lenses. When you get done crunching the numbers you find that the final result -- the physical manifestation of diffraction on the film -- is dependent only on the f/stop, and the focal length and absolute diameter of the aperture make no difference. A 20mm lens at f/22 will create the same size Airy disc as will a 200mm lens at f/22. In short, all lenses will produce the same amount of diffraction when set at the same f/stop.

http://photo.net/large-format-photography-forum/00JsZ7

So of this is correct, it's in a sense true that diffraction depends only on the absolute diameter, but the effect of diffraction on resolution is dependent only on the f stop.