Two men aged 23 and 52 were arrested at an address on Twingate in Rochdale. A 51-year-old man was arrested on Whittaker Lane in the town, while a 44-year-old man was arrested on Kingsway in Manchester.
All four were arrested on suspicion of conspiracy to supply Class A, B and C drugs and remain in custody.
Police said they believe up to one million tablets were recovered during the operation, which targeted properties linked to suspected drug storage and distribution.
Greater Manchester Police executed six warrants as part of a six-month investigation led by its Serious Organised Crime Group.
McCorry said officers used surveillance, drug seizures, phone analysis and automatic number plate recognition data to build evidence against the suspects.
One of the properties searched off Manchester Road is believed to have been used as a drug safe house.