If you're measuring customer loyalty by visit numbers only, regardless of the number of visits per month, then you can create Segments using the Visit Number dimension.
But if you need it to be visit numbers per month, then I don't think that is something that can be done using AA's out-of-the-box dimensions, metrics and functions. The best that I can suggest is a segment like this:
Name: Loyal customer
"Last month" date range
"Visits" greater than or equal to 4
The limitation with this segment is that regardless of the date range of your dashboard/report/project, AA will always return the data for the last month of the current day because of that date range definition. So given today is 6 December 2020, your dashboard's date range could be 1-5 December 2020 or 1-31 October 2020 or 1-31 December 2019, but AA will always return the same data for 1-30 November 2020.