The study was due to begin on 01 November, however changes on how the Department of Transport carries out its methodology means the new inquiry will not start until 31 March 2017.
The delay will give the Welsh Government more time to assess new data which forecasts vehicle traffic growth in the region.
The £1 billion project involves building a new three-lane section of motorway between Magor and Castleton to the south of Newport as well as a 440-metre main span cable-stayed bridge.
Complementary measures include new cycling/walking infrastructure, improved road connections and reclassification of the existing motorway.
The design development team consists of Costain, Vinci and Taylor Woodrow with consultants Arup and Atkins, supported by environmental consultants RPS.
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Cabinet Secretary for Economy and Infrastructure, Ken Skates, said despite the delay, the project will still be completed in 2021.
"The UK Government has already expressed its desire to see work on the M4 Project start as soon as possible, so it is hugely disappointing that the Department for Transport chose only to engage with us on these amendments to their traffic forecasting three days before they were introduced forcing this regrettable delay to the start date of the public inquiry," he said.
"Whilst we push forward with our Metro plans for north and south Wales, the long standing and continued problems associated with the M4 around Newport need a solution that delivers a long term, integrated and sustainable transport system.
"Assessments indicate that our proposed M4 Project is the only reasonable solution, but it is only right and proper that the proposals are scrutinised robustly, transparently and openly in an inquiry and that we test the project in light of the most up-to-date information available to ensure that we deliver the right choice for the people in Wales.
"It is essential that we, and others who wish to submit evidence to the public inquiry, now take time to fully review these new Department for Transport figures before the inquiry begins."
Work on the scheme is due to begin in Spring 2018.
(LM)
