Currently Being ModeratedJan 9, 2013 2:38 PM (in response to cbt19350)
You can repeat it every month by tapping Repeat when composing the event. If you want it to repeat every 4 weeks you would have to use an app that supports this option such as Week Calendar.
Currently Being ModeratedJan 9, 2013 2:45 PM (in response to cbt19350)
Your question led me to discover something interesting ... if you have a Mac and use iCloud calendars.
Randers4's answer is absolutely correct. But if you have a Mac and use an iCloud calendar, you can in fact set up an "every 4 weeks" event on the Mac and have that carry over to your iPhone. Simply open the even in your Mac's calendar, and choose a "custom" repeat interval. You can choose "every 4 weeks" there. In fact, I've just tried it and it does carry over to your iPhone.