macboy4hak,
Probably not the solution that you're looking for, but I do also have a need to be reminded every four weeks, not every month. I want to be reminded to pay rent/utilities on the day that I get paid (which is every other week, not twice a month).
Here's what I did (and I realize that this solution may not be available to you): I asked Siri to remind me to pay rent every four weeks. It sets up the custom reminder for four weeks from today, and I then I went in and changed the date to the day that I wanted.
Again, not a perfect solution, it might not be available to you, and you're absolutely right that it should be an option built in to the reminders app, but it's a workaround until the feature is added.
mastr K