There are ways to do this with the regular reminders app both for the last day of the month and for every 4 weeks.
For the last day of the month:
- Create your reminder
- Click the "i"
- Select Remind me on a day
- Select Repeat
- Select Custom
- Under Frequency select Monthly
- Under Every select 1 Month
- Select On the...
- On the left scroll to "last"
- On the right scroll to "day"
For every 4 weeks (or whatever weekly interval you would like):
- Create your reminder
- Click the "i"
- Select Remind me on a day
- Select Repeat
- Select Custom
- Under Frequency select Weekly
- Under Every select 4 weeks
- Select which day of the week you would like it to repeat