Climbing to Good English [R&S?] is 4th.
I think your ds would do better and learn more if he did 5th grade before 6th and so on. It wouldn't be such a jump. We used some R&S before going to Hake. Only my strong grammar son would have done well skipping a level. There is more practice and review on some topics in the lower level and merely a quick review in the next level. Building, similar to Saxon math.