Authorities are being tightlipped about the arrests in the west coast city of Goteborg, which came on the eve of the 9/11 anniversary, and were followed by the evacuation of about 400 people from an arts center.
Swedish news agency TT said three of the suspects are Swedish citizens, two of whom were born in Africa and one in the Middle East. The fourth suspect is an African-born man with a residence permit in Sweden, TT said.
The Swedish Prosecution Authority said in a statement that investigation leader Agnetha Hilding Qvarnstrom must decide by Tuesday whether to release the suspects, or seek court approval to keep them in custody.
Prosecutors and police won't say whether the suspects are suspected of links to Islamic terror groups or other extremist networks. The prosecutors' statement said they were arrested without incident and noted that "the investigation is still in an early stage."
Ulf Ahlstedt, who has been appointed as defense lawyer for one of the suspects, said his client denies the allegations, but declined to give any other details.
"What I can say is that he's a man and he is suspected of preparation to commit a terrorist offense, which he denies," Ahlstedt told The Associated Press.
Sweden's security service SAPO said the Goteborg police and a national police anti-terror unit were involved in the arrests Saturday.
Goteborg police said they had evacuated the Roda Sten arts center, located beneath the city's landmark half-mile (930-meter) Alvsborg bridge, because of a threat deemed to pose "serious danger to life, health or substantial damage of property."
The Alvsborg bridge runs over Goteborg's harbor to connect the mainland with the island of Hisingen. The six-lane passage is a vital link from the Norwegian capital Oslo to southern Sweden.
SAPO wouldn't specify the target of the alleged plot, but said after the arrests that there was no reason for public concern and maintained its terror alert at "elevated." It has remained at that level since being raised from "low" in October.
Swedish tabloid Expressen, citing an unnamed police source, said investigators suspect the men belong to a terror network with links to al-Qaida, and that they had acquired, or were trying to acquire, firearms, explosives and hand grenades. Security police declined to comment on the newspaper's information.
Once peaceful Scandinavia is no longer insulated from terrorism. Danish authorities have foiled a series of plots against a newspaper that printed caricatures of the Prophet Muhammad in 2005, sparking riots in Muslim countries.
In December, suicide bomber Taimour Abdulwahab blew himself up in downtown Stockholm among panicked Christmas shoppers, injuring two people and causing shock in a country that had largely been insulated from terrorism.
On July 22, a self-styled anti-Muslim extremist blew up a car bomb in downtown Oslo and then massacred youth gathered for a Labor Party summer camp, killing 77 people in the region's worst violence since World War II.