In the constraint, you can constrain the question to be 1-4 or null. Then so long as you require the question when the skip patterns are such that it should be answered, this should solve your problem. If it’s required, it can’t be null so must be 1-4, and when it’s not required, it can be null.
I think we answered the dates validation question here : Is it possible to validate dates in odk-x - #3 by hissdev