The haul from Belgium's most spectacular diamond robbery in Antwerp earlier this month was worth more than 100 million euros ($107m), Belgian police have said.
Vault keys seized by police are displayed as evidence
In what police authorities have called the heist of the century, 123 of the 160 vaults at the Antwerp Diamond Centre were emptied.
Antwerp's judicial director, Erik Sack, called the crime on 16 February "a piece of genius in its simplicity".
He said the gang had learned to circumvent the alarm system and had copied master keys after renting an office in the Diamond Centre in the name of a phantom company.
The floor was strewn with safety boxes, gold, money, securities, cut and rough diamonds, jewels
Antwerp judicial director, Erik Sack
During the theft, they taped over security cameras
and may have put old videotapes in the
surveillance system, police said.
This gave them enough time to break open the vaults, but they uncovered so many riches they were unable to take them all away.
"The floor was strewn with safety boxes, gold, money, securities, cut and rough diamonds, jewels," said Mr Sack.
It is pretty clear that those who executed the crime came over from Italy and our investigation is now centring on them
The Diamond Centre is located in the heart of Antwerp's diamond district which is constantly monitored by police and dozens of cameras.
Special passes are required to gain access to the building, and guards protect the room with the vaults 24 hours a day.
"We'd describe it as a piece of genius in its simplicity, not least because the security system was so thoroughly analysed," said Mr Sack.
Police were led to the four arrested by a discovery at the side of a Belgian motorway.
Bags containing security-camera tapes and documents from safes had been carelessly discarded.
A suspect was then arrested when he returned to his office in the Diamond Centre last weekend, authorities said.
Three others were seized at an Antwerp apartment.
The four - three Italians and a Dutch woman - were remanded in custody on Thursday, as investigations continued.
Mr Sack said police believed the ringleader was among those already arrested, but that three people suspected of opening the vaults and grabbing the loot were currently being sought in Italy.
"It is pretty clear that those who executed the crime came over from Italy and our investigation is now centring on them," Mr Sack said.
The diamonds have not yet been recovered.