It might be because ejection would be at a low altitude even with the -4. You could email Art and ask. I personally wouldn't hesitate to try an E9.I had 7.25" Original Flying Saucer. It was my most reliable rocket for a good flight on an E9. I flew it at least once at every launch for years. So I was a little surprised not to see any E9s in the recommended motor lists for the Ultra Delta or Super Cinco.