Hi Pranitha,
You can achieve this even with report by execution for work item creation = sy datum - 4 days and for all multiples of three days from (sy-datum - 4 days) until the date range you provide in the selection screen of the report.
If you implement program exits it will be more easier where you can have one field in the custom table to store last reminder date and for request status. based on these table values you can directly send reminders.
Regards,
Jayaraj Chembra