Prensa Libre, a Guatemalan newspaper, confirmed the release and said the two locals have also been freed.
The indigenous farmers' group told Guatemalan radio on Saturday that the six hostages would be held until Alvaro Colom, Guatemala's president, agreed to talk to them.
But Luis Chol, a member of the farmers' group, said they were forced to release the Belgians because they were being attacked.
The police "followed us and attacked us and killed one of our comrades", the Associated Press quoted Chol as saying.
The government denies anyone was killed.
The Belgians were identified as Eric and Jenny Stosstris, and Gabriel and Mary Paul Van Huysse.
Robles said they were exchanged for three farmers who had been arrested by police on Saturday.
The farmers are demanding the legalisation of their land claims and freedom for Ramiro Choc, who was arrested on February 14 on charges of illegal land invasion, robbery and holding people against their will.
They contend they have lived on the disputed land for more than a decade and that they are now being threatened with eviction.
The farmers' group last month captured 29 Guatemalan police in the Caribbean coastal town of Livingston.
The police were held for almost two days before being releasing in exchange for similar assurances from the government to address their claims.