Ahead of new powers to raise cash in 2018, the UK government is allowing Welsh ministers to borrow £500m for the proposed relief road.
But Jenny Rathbone said the benefits of the metro project to upgrade public transport in south Wales were greater.
The Welsh Government said it will deliver both schemes.
A Labour colleague of Ms Rathbone, John Griffiths, also called for the cash to be spent on the metro, saying it had more political support in the assembly than the M4 relief road.
Recent assessments have put the cost of the M4 upgrade at £1.1bn, excluding VAT and inflation. One estimate of the overall cost of the Metro, which could see the use of trams, has been put at £2bn.
Some £764m has been allocated for the Metro as part of the 20-year Cardiff Capital Region deal and EU funding could also be used.
Ms Rathbone, who has expressed her opposition to the road scheme before, told BBC Wales: "We don't have the money to do both."
She said the metro project "needs more money to do it properly" to ensure main valleys communities are linked.
"I'm not aware of any other pot of money that we could use to generate this amount of money. It does not exist," the backbench AM added.
"We can't get EU money for the M4. We have some EU money for the metro and but not all the money that is needed."
The Cardiff Central AM said she believed pressing ahead with the metro "would resolve the congestion problems we have got on the M4", which she said arise from the "100,000 commuters that come into Cardiff and Newport every day".
The Welsh Government will be able to borrow up to £500m from 2018, but ahead of those powers coming into force the UK government has allowed for that money to be borrowed early for the M4 project.
Ms Rathbone said it should be "up to the National Assembly to decide how to use that early borrowing facility".
A spokeswoman for the Welsh Government said: "Both the M4 relief road and the metro are hugely important to our vision for a fully integrated transport system for Wales.
"This Welsh Government was elected on a mandate to deliver both of these ambitious projects and that is what we will do.
"Our spending plans for future years will reflect our priorities for this administration."
Money allocated for the M4 relief road should be spent on the metro for south Wales as money is not there for both, a Labour AM has warned.