The men have been trapped by water at Gleision Colliery near Cilybebyll, Pontardawe in the Swansea Valley since Thursday morning.
No contact has yet been made with the miners but emergency services are pumping water from the mine.
Two men managed to escape from the mine when it flooded and a third came out about an hour later and is in Morriston Hospital in a critical condition with internal injuries.
Chris Margetts, from South Wales Fire and Rescue Service, said: “What we have determined is the miners are located approximately 90m underground.
I know at least two of the four and I’m quietly optimistic, given that they might have found a safe haven this morning, that they will still be there”
“They are down a 250m main route into the mine… there are numerous little tunnels and old workings which all potentially have air pockets in.”
“They are experienced miners, they know the layout of the mine, they would know where to go in this situation.”
“What we are dealing with is their egress out of the mine is full of water.”
He said conditions in the mine were favourable and he was “very hopeful and optimistic” the men could be freed successfully.
Rescuers were using a parallel tunnel to pump oxygen into the mine while also attempting to pump water out.
Two of the miners involved were father and son, with the son escaping earlier while the father was still trapped.
It is thought the men became trapped by water after breaking into old mine workings.
A coal mine still open in Wales? Must be the one that Margaret Thatcher missed.