Police seized more than 3kg of heroin, 20kg of cannabis and €40,000 in cash in an anti-drugs operation on Thursday night.
The operation began in Santa Lucija, when officers stopped and searched two vehicles. Inside one, a van, they found packets of suspected cannabis.
A search of a car nearby yielded more than €40,000 in cash.
They then proceeded to search a house in Żebbuġ and found 3kg of heroin hidden inside.
Three Maltese men – two from Cospicua and one from Żebbuġ – and one Sicilian who lives in Marsa were detained and arrested.
In a statement, the police said the four suspects, aged between 26 and 50, were helping them with the investigation but could be arraigned and charged with drug trafficking, aggravated possession and money laundering.
An inquiry is being led by magistrate Caroline Farrugia Frendo.