A total of seven young men between the ages of 19 and 23 are being held in custody for the rapes.
The men are believed to belong to a network of several smaller groups who have systematically raped young women and girls in the town.
Six of them have been charged and four of them stood trial in Södertälje district court on Thursday for aggravated rape, among other charges, in one of the four cases.
The girls have said the men took turns raping them while the others held them down, newspaper Länstidningen Södertälje reports.
“We have good reason to believe that several more women have been raped and we implore them to contact us,” Kia Samrell, information office at the Södertälje police department, told TT.
The case began in February when a young women reported that she had been raped by four men in an apartment in Hovsjö. Due to DNA evidence and information from various witnesses, the attack was linked to three additional unsolved rape cases in Södertälje.
One of the cases involved a woman who at the beginning of February believed she was getting a ride home, but was instead taken to a camping site where she was raped by several men.
In March last year, another young women was forced into a car in Lina and raped by five men. In the fourth case, which occurred in November 2007, a 12-year-old girl was raped by three men in an attic in Ronna.
The individual suspects have not all been involved in each rape.
“The men belong to different overlapping constellations,” said Samrell.