I have been traveling and hiking with a Toyo 810G for five months now. To pack it in a backpack I got a Toyo short extension rail (150 mm/6"), long enough for both standards but no more. This allows for the camera to lie flat in my Supertrekker backpack (which weighs in at about 25 kgs). Unpacking means attaching one more rail while the camera is still in the pack and extending the front standard. This method should work well for a 4x5 too.
I think there are some very fine 4x5 field cameras. For 8x10, however, the bed extension design just is not stable enough, especially with longer lenses. The same can of course be said when using very long extension with a 4x5.