Hello. Yes, I think you can but it may depend upon how you want to calculate an 'average'. Meaning say you define the 'average' as total visits divided by 28 days. Then yes, a calculated metric can be programmed to take total visits divided by 28. You would then have to apply a 'segment' that only grabs data from the past 4 weeks, rolling weekly. Adobe has a predefined Last 4 Full Weeks time segment, you just need to convert that into a regular Segment, because you can't use Time segments in calculated metrics, but you can use regular segments.