Hun Sen ordered the four men to be released from prison but will not be dropping the charges of criminal defamation against them.
The action came during Hun Sen's meeting with Christopher Hill, the US assistant secretary of state for East Asia and the Pacific, who is in Phnom Penh to mark the opening of a new embassy compound.
Om Yentieng, a prime ministerial spokesman, said: "Prime Minister Hun Sen is aware of those concerns and to demonstrate his compassion, he is sending a letter to the court ordering that those being detained are released on bail. This is a gift for Mr Christopher on the day of the inauguration of the new embassy."
The US has been critical of the arrests of the four critics. There are concerns that Cambodia's fledgling democracy is lurching towards dictatorship once more under Hun Sen, the former Khmer Rouge soldier who has run the country for 20 years.
Among those being freed are Kem Sokha, a vocal critic of Hun Sen and head of the US-funded Cambodian Centre for Human Rights (CCHR), and his deputy, Pa Nguon Teang.
Both men were charged with defamation in connection with a banner at a human rights rally in December that said Hun Sen was a communist and a traitor who sold off land to neighbouring Vietnam. They could be jailed for 18 months if convicted.
The other two are Mam Sonando, director of the independent Beehive radio station, who was arrested after airing comments on a border deal that Hun Sen signed with Vietnam in October, and Rong Chhun, the head of a teachers' union who also commented on the deal.
Another human rights worker affiliated with the CCHR, Yeng Vireak, was released on bail last week.The US state department said the cumulative effect of Hun Sen's use of defamation suits against critics and opponents called into question "the Cambodian government's commitment to democracy and human rights".
Washington led expressions of international disapproval of the arrests, which came shortly after an 18-month jail sentence was handed down in absentia to Sam Rainsy, the opposition leader who is in self-imposed exile after losing his parliamentary immunity.
"This is a gift for Mr Christopher on the day of the inauguration of the new embassy"
Om Yentieng, PM's spokesman
He had accused Hun Sen of trying to assassinate him in a grenade attack on an opposition rally that killed at least 16 people in 1997.