Police also identified the four victims' nationalities -- two were Swedish, one British and one Belgian, while the prosecutor's office said a second arrest had been made.
The first suspect, a 39-year-old man from the central Asian nation of Uzbekistan, was known to intelligence services before he was arrested on Friday.
"We know he has shown sympathies to extreme groups, among them ISIS. We won't comment any further on that," national police spokesman Jonas Hysing said.
The suspect had applied for residency in Sweden in 2014 but his application was rejected in December last year, another police spokesman, Jan Evensson, told reporters.
He was given four weeks to leave the country, and in February his case was handed to police to deport him.
Karin Rosander, spokeswoman for Sweden's prosecutors office, said that the second person arrested was "suspected of terror crimes through murder."
"He is suspected on reasonable grounds, which is the lower degree of suspicion. The other individual is suspected with probable cause, the higher degree of suspicion," she said.
Britain's foreign office named UK citizen Chris Bevington as among the dead, but gave no details on the victim.
"We are all devastated by the untimely and tragic death of our talented, compassionate and caring son Chris," the victim's father, John Bevington, said in a statement. "A wonderful husband, son, father, brother and close friend to many."
Bevington worked as a director for the music streaming service Spotify.
"The only light in this deeply tragic moment is the outpouring of love, compassion and solidarity that we have seen from everyone," said Spotify founder Daniel Ek on Facebook
. "And that was exactly the kind of person Chris was as well."
Truck attacks strike Europe
The attack was launched in Stockholm on Friday when a beer truck plowed into a department store in the center of the Swedish capital, sending crowds running in panic. In addition to the four killed, 15 people were injured, four of them critically, Stockholm County Council said earlier.
Sweden has stepped up its security, with approximately 100 police officers deployed from across the country to assist in Stockholm. National counterterrorism, bomb and air assets are also providing support.
The incident comes on the heels of several similar attacks in Europe -- a man rammed his car into a crowd in London near Parliament in March, killing five people. In December, a large truck plowed into a Christmas market in Berlin, killing 12. And on France's Bastille Day last July, at least 85 people were killed when another large truck mowed down a crowd celebrating the day.
Swedish police said Saturday that they had found a "device" in the truck that they were investigating.
"Whether this was a classic bomb or some sort of flammable device is now a matter for our analysis," Swedish Police Chief Dan Eliasson said.
Sweden's public broadcaster SVT had reported that a bag of undetonated explosives was found inside the truck, which was stolen minutes before the attack as it made a delivery at a restaurant. The attacker apparently suffered burns caused by the explosives, which did not detonate properly, SVT said.
A manhunt was launched immediately after the attack. The suspect was detained Friday in Marsta, a town about 30 minutes' drive north of Stockholm near the city's main international airport.
'This will change things'
Thousand of people gathered at Stockholm's Sergel Square on Sunday to remember the victims. The organizers called on people to put their political views and differences aside, and come together in a show of unity.
Attendees laid flowers on a large Swedish flag placed down the square's steps, while a woman in an Islamic headscarf was seen smiling with police guarding the ceremony, as she handed them flowers.
"Sweden is united," said Stockholm resident Olsa Ekermann.
"This will change things though. People are more alert, and earlier I heard an ambulance, and everyone looked afraid like something was happening."