More than 100 officers raided the Cariad Farm Peterstone in Marshfield and two other locations Monday, the South Wales Argus reported Tuesday.
An Eastern European man was taken from the farm Monday and hospitalized after police said he was found in "very poor conditions."
The man, believed to be from Poland, remained hospitalized Tuesday. Officials said his physical and mental conditions were being evaluated.
Police also said a body may be buried on the farm and have dispatch specially trained officers and a forensic archaeologist to the scene.
Police said they expected to be digging at the farm for about three days.
Those arrested include two men, ages 66 and 42, from the Peterstone area, a 36-year-old man from Cardiff and a 42-year-old woman from the Penhow area.
The investigation began six months ago after a 43-year-old man, Darrell Simester, reported missing 13 years ago was discovered living in a run-down van in Peterstone. He told officials he had been kept as a slave by travelers.
A neighbor said the family at the farm kept to themselves. While the family had lived in the area "years and years," the neighbor said Tuesday "we don't know a lot about them,"
A Home office spokesman said the incident was "an appalling reminder of the extent to which slavery has reappeared in our country."
The Home Office will introduce a bill to deal with modern-day slavery to this session of Parliament, the spokesman said.