The incident occurred eastbound between Junction 25 Caerleon and Junction 24 Coldra, with long queues and and major delays going back as far as Junction 29.
An air ambulance was flown in with the motorway closed in both directions near Newport on Saturday morning (September 24).
All traffic was held in both directions, but westbound re-opened at 12.17pm.
Lane three was later opened eastbound to release trapped traffic.
Gwent Police had earlier said: "The M4 is currently closed in both directions between Junction 24 (Coldra) and 25 (Caerleon). Emergency services are currently at scene dealing with an incident. Please find an alternative route."