Detective Chief Inspector Dean Taylor said: “We received concerns about the conduct of staff at HMP Parc on Wednesday September 18.
“Officers are at the early stages of the investigation and are working closely with G4S while inquiries are ongoing.”
The four arrested are a 23-year-old woman from Bridgend; a 45-year-old man from Pontycymer; a 25-year-old man from Merthyr and a 35-year-old man from Llanelli.
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