my exercise routines occur every 3rd day and every 4th day.
would like a way to specify that
unfortunately, it's not possible to set a due date for every 3rd and 4th day of the month at the same time, but we will consider adding such option in future development. You're welcome to add your suggestion or your vote for this request in our Votebox: http://todoist.com/Vote/showProposal/22/
For now, you could set up 2 separate tasks - one with a due date: "every 3" which would set it to every 3rd day of the month and one to "every 4".
If you're referring to a task recurring every 3 days, though, please use "every 3 days" as a due date.