The operation was the culmination of weeks of surveillance and intelligence-gathering by detectives working closely on both sides of the border.
Last night, three of the arrested people were still being questioned in the Northumbria area; the fourth was being held in the Lothian and Borders force area. None had been charged.
Detectives involved in the investigation had said earlier that they believed the thefts from the six premises were the work of a gang who appeared to be stealing to order. Many valuable works of art were bypassed while others were taken.
Operation Border Reivers - an old Scottish word for raiders - began at 7am yesterday when police from Northumbria, Tayside, Cumbria, and Lothian and Borders searched homes in the east, west and city centre of Newcastle, and the coastal area.
Two people were arrested in the early raids, while another two were seized later in the morning. Considerable amounts of property were discovered at various houses.
Among the items found were silverware, including snuff-boxes and candelabra, jewellery, clocks and lacquered papier-macheware. But police said it was too early to say whether the items came from the six premises that were burgled.
The country houses and castles - Bamburgh Castle in Northumberland; Abbotsford House in Melrose; Sizergh Castle, Cumbria; Scone Palace, Tayside; Floors Castle in the Borders, and Levington Hall in Cumbria - were all robbed within the space of a few weeks.
The most recent raid took place at Scone Palace, the home of the Earl of Mansfield, on 25 May. The thieves are believed to have carried out the robbery in just four minutes, ignoring many valuable items and going straight for a glass case of papier-mache Vernis Martin lacquerware. In another glass case, they found a unique collection of Bavarian ivories. The Earl estimated the value of the stolen property at up to pounds 500,000.
In the raid at Floors Castle - home of the Duke of Roxburghe - the thieves rowed across the river Tweed in a dinghy before making their way over parkland to break in and steal Faberge and Cartier items worth a six-figure sum.
Abbotsford in the Borders - former home of the 19th-century novelist Sir Walter Scott - was raided last month. Napoleon's cloak clasp and Bonnie Prince Charlie's whisky flask were among the items stolen.Reuse content