Gonna pile on to other's suggestions, and say Todoist can do this. You can schedule a task to happen every four weeks, and the next occurrence happen four weeks

Gonna pile on to other's suggestions, and say Todoist can do this. You can schedule a task to happen every four weeks, and the next occurrence happen four weeks after you complete it (even if you're a few days late) or four weeks from when it was last due. Pretty nice for my needs, might fit yours too.