The variable speed limit camera system was set up five years ago.
It originally aimed to keep traffic moving between junctions 24 at the Coldra and 28 at Tredegar Park.
But in July the Welsh Government confirmed it would target those who "flouted" the limits and GoSafe, who will lead it, said the move was not about prosecuting "for the sake of it".
A spokesman for GoSafe - a police and council speed camera enforcement partnership - said the public would have plenty of warning before it starts.
The system is made up of electronic signs which display speed limits, together with traditional road signs indicating that enforcement cameras are in place.
Prosecutions can be handed out whatever the variable limit is from 20mph to 70mph.
Prosecutions against drivers who break the variable speed limit on the M4 around Newport will begin "imminently".