The four men detained over last week's attack on a State Duma deputy face charges of robbery and extortion as part of another criminal case, the Investigative Committee said.
Investigators suspect that Khachubek Khadisov, Artur Minbulatov, Zakarya Gadzhiyev and Shamil Gadzhiyev attempted to extort 30 million rubles ($1 million) from a Moscow businessman in March.
"They threatened the victim and his family members [with violence]," the committee said in a statement. "After the businessman tried to call for help by phone, they beat him and took his cellphone."
The four were detained last week near the city of Volgograd, 900 kilometers southeast of Moscow. Two of them were arrested and face charges of hooliganism and assault and battery over last week's attack on LDPR lawmaker Roman Khudyakov.
Khudyakov was beaten in an apparent road rage attack by two men while traveling home from work by car on July 10.