Staffordshire Police said warrants were executed at four addresses in Manchester and the Midlands between 6.30 and 7.00 this morning.
Three men and one woman were arrested on suspicion of conspiracy to blackmail David Hall and Partners, a family-run guinea pig breeding farm which recently announced it would close at the end of the year following a six-year hate campaign by animal rights extremists.
The family said at the time that they hoped their decision would prompt grave robbers to return the body of 82-year-old Gladys Hammond.
Mrs Hammond was the mother-in-law of Christopher Hall, who co-owns Darley Oaks Farm, and her body was stolen from a grave at St Peter's Church in nearby Yoxall last October.
Police arrested a 35-year-old man in Manchester, a 36-year-old man in the Edgbaston area of Birmingham, a 38-year-old man in Wolverhampton, and a 37-year-old woman in Burntwood, Staffordshire.
All four were taken to a police station in Staffordshire for questioning.
A 23-year-woman was arrested at the Wolverhampton address on suspicion of obstructing police and assaulting an officer and taken to a police station in the West Midlands.