I am dealing with slightly underprocessed hair as well, only on the left side. I plan on just dealing with it. I feel like it is better to be a little underprocessed, because that part can get straight later. If you overprocess, then there's no going back. If you aren't having any breakage and the hair isn't too tough, I would try to go at least 6 weeks before relaxing again. But at a minimum, I don't think you can relax again before 4 weeks.