The short answer is - you're going to get this from the updated field. That's a straight date field and is checking calendar values against that field value to get a numerical difference.
My first thought is to create your own 'diff' field and maintain that yourself. Run a scheduled job daily (perhaps hourly) to find records that were not updated in the last 4 hours and increment the "day" value on that field if it has not been updated on a business day. You can then use your report to check for field values > 2 in that field. It doesn't have to go on a form to be used, but it does have to be on the table somewhere so the script (and report) can reference it.
For what it's worth, this is a bit of a customization for a single report. Make sure whomever is request this understands the development effort involved - it's not available OOB and will require updates, testing, etc. in the future. Adding cost (and time) to this requirement. Be sure they are willing to fund it and accept the ongoing risks/costs.
Perhaps someone else will jump in with a simpler solution, but I don't have one at this point.
