Here is a good link for information and pictures about 4 vessel umbilical cords:
There are some articles that suggest that a 4 vessel cord is correlated with multiple congenital anomalies. These should all (or most) be visible on ultrasound. I could also find several case reports of normal infants born with 4 vessels in the cord. It depends a lot on whether the extra vessel is a vein or an artery, with arteries being much less significant. In fact up to 5% of umbilical cords have an extra vein at some point in their length.
I hope it helps
Good luck xxx