The GBC Zook Hero 3 is a Sintax game though, and it has a different (but similar) name in Chinese, 洛克英雄传3 (Rock Hero Legend 3, or something), so its kind of an unofficial sequel - the GBA Rockman & Crystal is the "real" Vast Fame-developed sequel as far as anyone knows.
incidentally Sintax also has copyrights registered on "洛克人 X5/Rockman X5" & "络克英雄EXE5/Pockman exe5", so there might be two more out there.. but knowing Sintax they'll just be two of their older games with rockman sprites hacked in.