Is four millimeters normal size for bile duct?

No, four millimeters is not considered a normal size for the bile duct. The average diameter of the common bile duct is usually between 6-8 millimeters. A bile duct that is 4 millimeters or less in diameter may be considered narrow or strictured, which can lead to problems with bile flow and potentially cause symptoms such as abdominal pain, jaundice, or itching.