I'd like to start with some troubleshooting to isolate the issue and see if it's a browser-related caused behavior.
The best way to handle missing information on forms that you've entered into your account is to clear the cache and cookies of the browser you're using for QuickBooks Online. These files can become outdated, causing issues like the one with your Employee's W-4 line 4C not showing up. Here's a link to help you with the process: Clear Web Browser History.
Once finished, try to look at the box on the form again and see if it populates. Let me know how this goes for you! I'll be on standby.