Saigo no Yakusoku

Janitors Satoru Mashiko (Satoshi Ohno) and Haruo Iio (Kazuki Kosakai) are driving towards Energy Bio Corporation for work when motorcycle courier Nozomu GotÅ (Jun Matsumoto) passes by them. GotÅ goes to deliver a package to Yuriko Niimi (Meisa Kuroki), the daughter of the president of Energy Bio Corporation. Meanwhile, Fuyuko Todoroki (Nene Otsuka) orders a cappuccino from coffee shop employee Akira Tanada (Masaki Aiba). They talk before she has to leave to meet with insurance salesman Yukio Tomizawa (Sho Sakurai) downstairs. As Mashiko and Iio start cleaning, Tanada apologizes for giving security center employee ShÅ«ji Yamagiwa (Kazunari Ninomiya) and new security manager ShinichirÅ Okanaka (Naohito Fujiki) the wrong order.
At 3:00Â p.m., a disguised man with a rifle informs everyone that he and his allies have hijacked the building and set up a bomb that will go off in ninety minutes should President Niimi (Masahiko Tsugawa) not arrive with the ransom. Before she is locked in with Tomizawa, Fuyuko and the rest of the hostages in the fifth floor lobby, Yuriko runs off with GotÅ running after her. At the police station, Niimi and chief detective Genji Morozumi (Yukiya Kitamura) keep in contact with a hiding Tanada.
Meanwhile, Yuriko quickly collects company data worth more than the ransom money. She reveals how she believes her father will only come to save his company. In turn, GotÅ reveals his own past of how his trusted friend died ten years ago. They cut off the power, which allows most of the hostages to escape the lobby. Okanaka distracts the hijackers to give Yamagiwa the chance to open the B-1 doors on the second floor for the hostages to escape the building after the power returns. Tanada informs the police that the basement doors are open, and Mashiko chooses not escape with Iio.
After the escaped hostages manage to get outside, GotÅ tells Yuriko to go while he tries to take down the hijacker chasing after them. The doors close in front of Yuriko, who stops when she hears a gunshot and sees GotÅ collapse; the hijackers take her away. Meanwhile, as the only two still trapped in the lobby, Fuyuko reveals to Tomizawa that her father died taking a drug handled by Niimi's company just before she is taken away as well. After administrating a background check, Morozumi finds out that the phone number Tanada has been using belongs to a young man who died ten years ago.
The hijackers take Niimi, who went in the building with the money, to the security room, where Yamagiwa, Okanaka, Yuriko and Fuyuko are tied up. One of the four hijackers in the room informs Niimi that all they want is for him to publicly admit that his company handled lethal medicine that killed their friend and Fuyuko's father. The hijackers reveal themselves to be Mashiko, Tomizawa, Tanada and GotÅ. Okanaka knew that there were only a total five hijackers, with Yamagiwa being the fifth one who oversaw the operation.
Although Niimi initially refused to admit that he distributed the drug while knowing the risks, he relents and Yuriko decides to stay by her father's side. Niimi apologizes to her and then confesses to the media. After Okanaka agrees to erase the security data of their kÅhai, who helped the five before the switch during the hostages' escape, the five escape through the air vents and into the basement. However, Mashiko realizes that he does not have the car key, only to find that Iio had left it on the car for him. At a distance from the building, the timer reaches zero and the bomb turns out to be an explosive for fireworks instead. Finally able to move forward, Mashiko, Tomizawa, Tanada, Yamagiwa and GotÅ readily surrender themselves to the police.

Question: Accordng to Okanaka's assessment,  how many hijackers are involved?
Answer: Five