No lift, change in tire/wheel size, gearing...nothing has been done to the drive train of the vehicle? The reason I threw out the idea to begin with is a slight change in mileage would be expected to the positive, but a 4 mpg loss represents around a 20% loss and I would suggest that it is an ECU issue and not necessarily an engine issue.
Have you run a JScan or another diagnostic to see what's going on? Highly recommend it.