Gwent Police said one vehicle towing another had jack-knifed into the central reservation on the eastbound carriageway between junction 23 and junction 22 just after 10:30 BST.
Two lanes heading towards the Second Severn Crossing were shut but have now reopened.
Queues have now eased.
Drivers were stuck in queues of up to 23-miles after two vehicles were involved in a crash on the M4 in Monmouthshire on Saturday morning.