Hi
What I think RLPope was suggesting was to set a number of rules on the hidden field that are conditional by the check boxes not being blank. For example,
Rule 1 Check-Box(CB) A is not blank and CB B is not blank and CB C is not blank and CB D is not blank Action Set a field's value, hidden field, concat(CBA, CBB, CBC, CBD)
Rule 2 CB A is blank and CB B is not blank and CB C is not blank and CB D is not blank Action Set a field's value, hidden field, concat(CBB, CBC, CBD)
And so on, this would then guarantee only the right rule will run which in turn guarantees the correct result.
Cheers
Adam