The staged water releases began last August in what Japan says is a key step in decommissioning the plant hit by an earthquake and tsunami in 2011 in the world's worst nuclear disaster since Chernobyl.
China and Russia have banned seafood imports from Japan over safety concerns that Tokyo has said are scientifically unjustified.
In the fourth release, about 7,800 cubic metres (275,454 cubic feet) of the treated water will be sent into the Pacific Ocean, similar to the previous three discharges, Junichi Matsumoto, the Tepco official overseeing the operation, said in a media presentation.
As part of the decades-long plan to release the water, the fifth and sixth discharges will occur during the fiscal year ending in March 2025, his presentation showed.
Japan says the water is treated to remove most radioactive elements except tritium, an isotope of hydrogen that must be diluted because it is difficult to filter.
Tritium levels in surrounding waters since the initial discharge have met pre-determined standards and are below the World Health Organization's guidelines for drinking water quality, Matsumoto said.
