Dinoshark

Eight teenagers (Ali, Pete, David, Mag, Luis, Kirby, Ron, and Dani) head to a secret island during Spring Break. Ron says that he and his family found the island when he was little, and there is no one else there. They immediately start partying. During the party, a drunken Luis decides to go skinny dipping. Ron tells the others the story of Dinoshark, a shark who was implanted with dinosaur DNA in an experiment, but broke free, killed the scientists experimenting on him and entered the ocean. Kirby laughs and dismisses the story as a myth, along with the others. However, Luis is eaten alive by Dinoshark.
Ron hits on Ali, but she turns him down, which makes him visibly angry. Dani dumps Pete for not supporting her in her stripper job, despite his pleas for her to forgive him. Dani then leaves the party to have sex with David after he gives her wine. She strips naked and convinces him to try a complicated sex position. However, when the position almost results in David getting injured, he storms off. After he leaves, Dinoshark leaps out of the water and grabs Dani's ankle. She tries holding onto a string of seaweed, but it ends up asphyxiating her and Dinoshark drags her corpse into the bay and eats most of it.
David and Pete climb a cliff while Kirby and Mag argue, and Mag insults Kirby for being a virgin. Ron and Ali break up the fight. Meanwhile, David decides to pee off the cliff, but loses his balance and nearly falls off. Pete manages to grab him, however. David then spots Dani's remains floating in the water, but he doesn't know what they are. David and Pete try and climb down the hill to get a closer look, but David trips and falls off. He lands in Dinoshark's mouth and is eaten whole. A horrified Pete scrambles up the mountain, but is knocked unconscious.
Mag walks off to sunbathe. She takes off her bikini top, exposing her breasts, and is noticed by Dinoshark. Ron suddenly attacks Mag and rips off the bottom of her bikini. He then ties a rope to her ankle and attaches the other end to a rock before throwing her into the ocean, using her as bait. Meanwhile, Pete awakens and returns to the party, finding that the only members of the group still at the beach are Kirby and Ali. Ron arrives back at about the same time, and the two announce the deaths of David and Mag.
The remaining four teens prepare to leave the island in their boat; however, Dinoshark smashes most of the dock and they flee in different directions. Pete discovers a still-living but badly mutilated Mag. She begs him for help, but Dinoshark appears and eats Mag, leaving just her leg. Pete notices how she was tied to the rock and remembers how Ron claimed she was dead, and begins to suspect Ron. Pete heads into the forest when he hears Kirby screaming, and at the same time, Ron meets up with Ali and the two head back to the boat, where he pulls a gun on her and forces her to strip naked.
Pete finds Kirby and discovers that she screamed because she had found the desiccated remains of a girl killed by Dinoshark in the beginning of the film. Pete tells Kirby that he suspects Ron while they return, but she does not believe him. However, when they arrive back at the beach, they see Ron shooting Ali in one of her breasts, knocking her into the water, where she is eaten. Ron tells Pete and Kirby that after his family discovered the hidden island, all of them were killed by Dinoshark. Witnessing their deaths fostered his lifelong obsession in violence, gore and death.
Kirby tells him that his relationship with Dinoshark will result in his death. Ron is disbelieving, and moves close to Dinoshark to prove her wrong. However, Dinoshark leaps out of the water and grabs Ron. He tries to pull Ron into the ocean, but his leg is caught on a rope attached to the dock; Ron is ripped in half and Dinoshark devours one half of his body. Pete and Kirby attempt to make it to the boat while he is distracted, but Dinoshark still attacks them. Kirby then grabs Ron's gun and shoots Dinoshark repeatedly. They start the boat, but Dinoshark reappears, so Pete hits Dinoshark with the boat, seemingly killing it. Kirby and Pete drive away in the boat, the only survivors of the eight. The camera then pans back to show the still-living Dinoshark eating the other half of Ron's body before the screen cuts to black.

Question: How many teens are remaining on the island?
Answer: 4