Why don't you use QLineEdit with a QIntValidator instead? You can probably also subclass the spin box and reimplement its paintevent to make sure no arrows are drawn.
Subclassing the spinbox and reimplement the paintevent sounds like a good solution.

Are you sure the company can't download 4.2.2? The licence is not tied to a concrete version of Qt...
It's not tied to a version? I have got the impression from my superiors that it is? I'll see if I can get some clarification in the matter.

Thanks for your answer wysota!