Using length of item number as condition in workflow criteria

I'm currently trying configure the following condition:

We have two types of part numbers; 4-digit and 5-digit numbers. I'm trying to set up a workflow condition where if there is a 4-digit number on the Affected Items tab of an ECO, one group is added to the worfklow whereas if its a 5-digit part, a different group is added. The expression I used for a similar search condition (Item Number Is Like 'pn-????') doesn't seem to work as workflow criteria. Can anyone provide feedback on if this can be done and how?

Thanks!