David Mulligan, 26, Fitri Yarasir, 52, Ahmet Taskin, 45, and Mustafa Oflu, 56, are charged with conspiracy to supply Class A drugs and the importation of a banned substance in Salford.
They are due to appear at Salford and Manchester Magistrates' Court later.
A search is continuing at an industrial unit on Cobden Street, police said.
Four men have been charged after £20m of heroin was seized at an industrial unit in Greater Manchester.