Police said he may appear via video link from hospital after he was shot in the leg and arrested on Friday.
It was the first time there had been a jihadi-style attack in Finland.
The four others will join the main suspect, Moroccan-born Abderrahman Mechkah, who police request be taken into custody for suspicion of two murders with terrorist intent and six attempted murders with terrorist intent.
His alleged crimes are being treated as murder and attempted murder with terrorist intent, a court in Finland said Monday. Police have not confirmed that.
"According to the tip-off, the suspect seemed radicalized and was interested in extreme thinking", it said in a statement, adding there was no information to suggest a threat of an attack.
The Turku Market Square on August 19, the day after the deadly attack.
The service said Mechkah was not among the around 350 people it monitored in its terrorism prevention program.
Officials are continuing to hunt for 22-year-old Moroccan national Younes Abouyaaqoub, who police say is the driver who plowed into the Las Ramblas thoroughfare in Barcelona, killing 14.
The Finnish intelligence agency SUPO said meanwhile that police authorities in southwestern Finland had received a tip early this year that Mechkah "had been radicalised and showed interest in extremist ideologies".
A Foreign and Commonwealth Office (FCO) spokeswoman said: "Our staff have offered support to a British man following an incident in Finland".
"We are investigating the role of these five other people but we are not sure yet if they had anything to do with [the attack]", said Det Supt Markus Laine, of the National Bureau of Investigation.