A 3.5 mile section of metal barrier between Junctions 17 and 18 has reached the end of its working life and will be replaced with a new stronger and safer concrete barrier.
The work will start on Monday, January 12 and will be completed by the end of March.
Highways Agency Project Sponsor Chris Pope, said:
This essential work will help to ensure that this section of the M4 remains safe and reliable for many years to come.
Every effort will be made to ensure any disruption is kept to a minimum and we would urge road users to drive carefully through the roadworks.
Three narrow lanes will be running throughout the main works and a 50mph speed limit will be in place for the safety of road users and our road workers.
Members of the public should contact the Highways Agency Information Line on 0300 123 5000.
Journalists should contact the Highways Agency press office on 0844 693 1448 and use the menu to speak to the most appropriate press officer.