"We are low paid workers. We've had a pay freeze for two years" says Dave Warren, from the PCS union.
"This year, there's a one percent increase available, which is a joke in itself, but apparently the Government wants to tie that to the loss of our terms and conditions. We find that totally unacceptable."
More top news
There will be a fresher feel with temperatures falling lower than on recent nights.
The accident between junction 24, Coldra, and 23A, Magor, has closed the eastbound carriageway.
Eloise Parry, 21, died after taking diet pills containing the poisonous Dinitrophenol (DNP).