Investigators said the four, three Romanians and one Turkish citizen, were taken into custody late last week.
Police said two of the Romanians and the Turkish national had been released from custody at the end of last year and were being tried at large for heroin possession. The two had been arrested in 2006 and the Turkish citizen was found to be the head of the drug dealing network. After release from prison, the two resumed their drug dealing activities.
The four were taken into custody by organized crime prosecutors and risk up to 25 years in prison if found guilty.