Do you mean the Kichflash from individual computers? That's flash memory that holds the OS and plugs into a zorro slot. If anything it could be faster than OS3.9 on an A4k, as the entire OS is on solid state memory (not just the kernal, a few libraries and things)
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That's only the A4k, and maybe the A1200 with a Zorro busboard. There's nothing stopping you installing OS4 onto a solid state IDE device on the A1, which should have similar results. Just remember that there is only a certain number of writes possible on those, so don't use it for swap if you enable virtual memory.
