A few quick comments:
1) On these 722.3 and 722.4 series transmissions vacuum is used to soften the shifts only. It has no control over whether or not they occur.
2) Google up "atsg 722.4" and you'll find a most helpful PDF that might help you figure out your problem.
Perhaps you've got something wrong with your K2 accumulator/clutch/etc. The PDF indicates that its used for 4th gear (as best as I can figure at least).
Not sure if it will help at all but Superior Transmissions has a spring kit for the valve body. Maybe a broken spring in the K2 accumulator can cause this? Maybe not.
This might be helpful too
http://www.atraonline.com/gears/2008/2008-07/2008_7_38.pdf
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