Fire crews were called to the incident involving two vehicles, between junction 24 at the Coldra and 25 for Caerleon, at 12:40 BST.
Appliances from Maindee, Malpas and Duffryn were sent to the scene.
Slow traffic was reported in the area but queues have since eased.
All lanes of the M4 westbound near Newport have reopened after an earlier closure following a crash which left diesel on the carriageway.