Monaco public prosecutor Jacques Doremieux said the last missing bag of jewels was found on Monday just across the border in France, in a street where a suspect was also detained.
Two other bags had been recovered in Monaco on Saturday shortly after the brazen theft took place.
Cartier is assessing the value of the theft, Doremieux said, giving a rough estimate of "several million euros".
"As of today, four arrests have been carried out: two robbers, an accomplice tasked with driving a stolen car, and a fourth person whose role is unclear but who probably came to get the driver out," Doremieux said.
A fifth person -- presumed to be "the other robber" -- was still being sought, he said.
The store, near Monaco's famous casino, was robbed in broad daylight.
The thieves dropped two bags in the street as police arrived on the scene. Video cameras later showed that a robber had made off with a third bag.
Doremieux said the suspects came from Vallauris, a town on France's Cote d'Azur near Cannes. They had no previous record for robbery, he said.
No one was hurt in the robbery, but the district around the casino was sealed off for hours to help investigators.
Armed robberies are rare in Monaco, a micro-state that is a playground for the wealthy.
In 2012, four Lithuanian men stole a collection of about 50 watches worth around 200,000 euros ($220,000) from a jewellery store. They were later handed jail terms ranging from five to 13 years.
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