Motorists who currently drive for free on the M4 will be paying a toll of up to $4 a trip when the road is widened in four years, the O'Farrell government announced on Thursday.

The widening of the M4 between Church Street, Parramatta and Homebush Bay Drive from three lanes in each direction to four will be the first stage of the WestConnex motorway project. It is expected to be finished by early 2017.

When it is finished, motorists will be charged according to how far they drive along the M4. The toll will range from about $1.50 to $3.90 in today's money, though the cost of that will increase with inflation.

The average toll is expected to be about $3, the government said in a summary of the business case for the WestConnex motorway, released on Thursday by Premier Barry O'Farrell and Roads Minister Duncan Gay, who were accompanied by Prime Minister Tony Abbott and Deputy Prime Minister Warren Truss.