Abstract: The competitive superiority of the 4-roll reducing technoloy over the 3-roll reducing technology is analyzed.A 4-roll simultaneously-driven reducing mill developed by some company is theoretically elaborated，focusing on such aspects as the drive principle of the internally/externally driven 4-roll stand，the fabricating and assembling of the roll stand and its precision gurantee system，the main stand and its drive system，as well as the roll stand changing device etc.