is there any way I can set up Google Calendar to remind me to pay rent
every 4th Monday (= every 28 days)?
Not monthly and every 4th Monday. That's why I missed my last payment ....
Every fourth Monday of each month:
The calendar will show you Summary: Monthly on the fourth Monday
The first step is very important since it will be the starting point and the event will repeat according to that particular day.
Every four weeks on Monday