The Health and Safety Executive report into Gleision in the Swansea Valley, seen by BBC Wales, details conditions in the mine when the incident happened.
The miners died when water burst into the area where they were working.
Last year, the mine manager was cleared of gross negligence manslaughter.
Charles Breslin, 62, David Powell, 50, Philip Hill, 44, and Garry Jenkins, 39, died when water flooded the mine after a controlled explosion.
The report said HSE investigators found two lengths of drilling rod, which indicated that advanced drilling had taken place in a technique used to determine the thickness of coal and the presence of water.
One miner, who had been working in that area of the mine the day before the incident, told investigators that water had been running from the bore hole "like a tap running half on".
The report also said a mine plan found in the manager's office clearly highlighted the area where the incident happened as having underground water.
A report into a mining accident in 2011 which killed four men has found evidence suggesting it was known there was a significant amount of water in the area they blasted into.