What does mild posterior ostcophytic ridging at C4-5 mean?

It means that you are beginning to have early arthritic changes in your cervical spine (or neck). There are 7 cervical vertebrae, so this is in the middle of your neck. Osteophytes are basically bone spurs, or a buildup of hardened bone material on the edges of vertebra that occurs when you are losing cartilage and the bones are rubbing against each other.