Agorwyd yr ymchwiliad ym mis Chwefror 2017, ac mae wedi gwrando ar farn y rheini sydd o blaid ac yn erbyn y cynnig i adeiladu darn chwe lôn, 14 milltir (23km) o hyd o draffordd i'r de o Gasnewydd, gan gynnwys pont ar draws Afon Wysg a gwaith mawr i newid cyffyrdd 23 a 29 ar yr M4.
Opened in February 2017, views have been heard from those for and against the proposal to build a new 14 mile (23km) six-lane section of motorway south of Newport, including a bridge across the River Usk and major remodelling of junctions 23 and 29 of the M4.