Why not use an E12-4 instead of an E9-4? Just saw one crash (built stock) using an Estes C6-3 off a 4 foot steel launch rod with no wind. I would use a D10-3 with no other alteration (or an 18mm D RMS).
We just had one crash on a C6-3 at our launch last Saturday as well. It went up about 30 feet then turned nearly horizontally toward the parking lot....
I agree with Fred that if you're going 24mm E an E12 would be better than the E9.