Hi Chris,
Lots of ways to tackle this one.
My favorite would be to use Targeted Auditing to capture the date time that the Project was switched to Current in a custom parameter, calculating a Target Completion Date that is 4 weeks later, then using the latter to filter for Projects whose Target Completion Date is < $$TODAY (but are not yet complete).
If you happen to have the functionality that automatically creates a baseline when a Project is set to Currrent "turned on", another approach would be to run a Baseline report for any Baseline whose Entry Date < $$TODAY-4w (and whose Project is still not complete).
Regards,
Doug