Short version: (Thanks Herbmeistr)
Despite having two premonitions and visions of everyone's deaths, Nick (Bobby Campo) is not able to save himself, his girlfriend, and their friend from being killed in the end. Everyone dies.
The racist man is killed by his own truck. The "MILF" is killed by a rock thrown by a lawnmower. George is diced by a CO2 canister forcing his body through a fence. Hunt is disembowled by a pool suction drain, and Janet is saved from a deadly car wash. They believe the chain is broken, until they find out that the Cowboy from the racetrack is alive, meaning the chain is still continuing. They find him just in time for him to get crushed by a falling bathtub. Outside of the hospital, George is killed by a speeding Ambulance. Nick makes it to the mall in time to save Lori and Janet. They believe the chain is finally broken. Fast forward a week, and they are all hit by a truck and killed immediately while inside a coffee shop. Every character that cheated death dies.
Really, really long version:
Like all the movies prior, one of the characters, Nick, has a vision. All of the main characters are at a racetrack watching a car race sitting in section 180 (which was the flight number of the plane that crashed in the first movie). One of the cars pops a tire thanks to a careless pit technician leaving a part loose on one of the cars, and it spins out. A chain of crashes happens, and eventually everyone is killed in various ways. Of course, it is all just a vision, and Nick wakes up before the events actually happen. He freaks out which causes all of his friends and a few people near by to leave and escape death.
The next day, all of the survivors return to the racetrack to visit a memorial of those that died. There, the racist man who escaped right before the crash confronts the security guard that survived. His wife died in the accidents, and the security guard, trying to save the mans life, wouldn't let him back into the stadium to save her. The racist man gets angry and calls the security guard a "nigger", then leaves when Nick confronts him for being racist.
Later that night, the racist man decides to get revenge on the black security guard. He is drunk, drinking Hice Pale Ale (which is also the beer that people drank in the second and third movie). He is a tow truck operator, and drove that to the black mans house. He gets out of the truck and pulls a wooden cross he plans to burn off of the back of the truck. He slips and causes his tow hook to hit the ground, but it misses him. He walks over to the man's yard and starts digging.
Suddenly, his lucky horse shoe drops from the rear view mirror and turns on his radio very loudly (ironically, it begins playing "Why Can't We Be Friends?"), which causes him to walk over to his truck. The beer he left on his dash board vibrates due to the music and falls off, hitting his parking brake, which makes his truck start moving. The sudden movement causes a gas can to spill behind the truck. The racist man is caught by his ankle in the hook and drags him down the street. The trail of gasoline is lit by the chain dragging, which lights on fire and burns him alive.
The characters watch the news and think it is just a freak accident.
Later, the "MILF" goes to the hair salon. As she is leaving, a passing lawn mower kicks a large rock out which goes clear through her eye socket and out the other end, killing her instantly.
The main characters, Nick, Lori, Janet and Hunt, begin to think something is up, and they google information about premonitions. They find the info regarding the first three movies and decide that it is really happening. They read that if the "chain is broken" and someone escapes their death, that they all will be saved from death. Janet gets freaked out and leaves, while Hunt says that if he's going to die, he's going to enjoy his last moments and "get laid".
Nick and his girlfriend Lori return to the rubble of the race track, trying to remember what order they die in so they can stop the chain. George, the black security guard, is still there watching the security camera for some reason, and takes them to the office where they can watch the security footage from the accident. There they learn that the order is Charlie, Hunt or Janet, George, then Lori, then himself. Nick suddenly gets another vision of a car and a CO2 canister igniting.
George, Nick and Lori rush over to the custom mechanic's shop to find Charlie. They find him inside and learn that his wife died in the accidents. He tells them he wants to be left alone so he can try and rebuild his life. Just then, a van accidentally is let loose from a lift, and rolls down the pavement. It is caught by a steel cable inches away from crushing George against a fence. He laughs and says "See? Guess it isn't my time to die yet". Right after he says that, the cable holding the van snaps and whips out, lashing open a CO2 canister, which launches out of the garage and hits George straight in the chest, forcing parts of his body through the fence, literally dicing him to death.
They then try to figure out if it is Hunt or Janet that dies next, but in the vision, they were both crushed by a falling piece of concrete at the same time. They split up. George and Lori go to find Janet while Nick goes to find Hunt.
Lori is going to a car wash because a bird pooped on her windshield and Hunt is "enjoying" himself at a golf resort pool.
Nick gets another vision while looking at an advertisement for "Clear Rivers Water" (Clear Rivers was the name of a character from the first and second movie) where water kills them both. He tries to call Hunt but leaves a message. Later, Hunt is listening to his messages, but a brat kid in the pool shoots his phone with a water gun and laughs. Hunt pushes the kid into the pool and steals the water gun, then puts it behind a fence onto electrical equipment, where it falls and triggers the pool drain.
Meanwhile, Janet is beginning to pull into the car wash. She ignores the warning that tells her to put down her car antenna before entering, and when the car wash starts, it snaps off and hits the electrical box. This causes the car wash to short out and lock her car in one place. Then, her sun roof malfunctions and opens only part way. Right after that the water main breaks and begins flooding her car with water. She thinks she is going to drown, so she sticks her head out of the sunroof to breathe, only to realise that her head is now moving straight towards flaps that buff the car, which will surely tear her head apart and kill her.
Hunt is still at the pool. He is relaxing on one of the chairs, flipping his lucky coin. A golfer gets a bad shot and hits Hunt in the leg with the ball, causing him to drop the coin into the pool, where it is quickly sucked into the drain. Hunt dives in after it, only to get sucked against the drain by his anus. After much struggling and nobody being able to see him, the pressure against his back side becomes too great and he is disemboweled by the suction. His intestines and other organs explode out of the drainage system, along with tons of blood and his lucky coin.
Back at the carwash, George and Lori arrive just in time, and ram into Janet's car, stopping it. They push it backwards and save her.
They explain that George is next and he doesn't believe that the chain is broken even though they saved Janet. He goes home and tries to kill himself.
Nick and Lori try to save him, but found out he has already hung himself. But, the rope breaks and he is okay. He explains that he has been trying to kill himself all night. First he ate a full bottle of pain killers, but just threw them up. Then he tried to gas himself using car emissions, but the car kept stalling. Now this. He finally believes that the chain was broken and they are all fine.
Days later, Nick is packing his bags to go to Amsterdam with Lori. While packing, he accidentally knocks the TV remote over which turns on the TV. Turns out that there was another survivor, the "cowboy", from the accident. The chain wasn't broken at all.
George and Nick rush to the hospital. On the floor above the cowboy, a war veteran is taking bath. The nurse forgets to turn the faucet off when he leaves the room, and the tub starts to overflow with water and soak the floor and the roof above the cowboy. He tries to press the call button, but it shocks him and ruins it. Though injured, the Cowboy tries to crawl away. Right when Nick and George enter, the tub falls from the ceiling and crushes the cowboy. The chain of deaths has started again.
Nick and George rush out of the hospital so they can protect George, knowing he is next. As they are crossing the street, George mentions something about feeling some odd deja vu... then is immediately hit by an ambulance. Nick realizes the race is on since either Janet or Lori is next.
As this is all going on, Lori and Janet are at the mall, shopping. They decide to go see the movie "Love Lays Dying". Nick arrives at the theater and gets Lori to follow him. Janet refuses to leave, claiming that if she was going to die she'd already be dead and that she's fine now. Just then, a chain of events goes off behind the theater in a building under construction, which causes the screen to explode and fill the theater with shrapnel, killing Janet. This triggers a chain of explosions all through the mall. Nick and Lori run to escape and get on the escalator, but it explodes. Nick grabs onto Lori, but slips, and she gets sucked into the gears and is crushed.
Suddenly, Nick realizes he's having another vision. Immediately after the vision ends, George is hit by the Ambulance and dies. He jumps into his car and races to the mall. Having seen the source of the fire in his vision, he runs to stop the fire instead of saving Janet and Lori. He runs into the room and pulls the fire alarm, but they haven't activated it yet, since the room is still unfinished. He finds a fire extinguisher and tries to put out the fire, but runs out of foam inside of it to put it all out. In his rush he accidentally knocks over a table, which knocks a nail gun off of the table, which shoots and nails his arm to the wall. A barrel of explosive stuff begins leaking and moving towards the fire. He reaches with his good arm and picks up a 2x4 and whacks one of the fire sprinklers and successfully puts out the fire. They all make it out alive.
Cut to a week later, and the three survivors, Nick, Lori and Janet, are all at the coffee shop, all 100% sure that they have broken the chain and are okay. Then, Nick looks around the room. He sees a bunch of things that killed other people. On the TV is a car race, on the newspaper is an ad for Love Lays Dying, and on the bulletin board is a poster for the park Hunt died at.
Nick asks "Hey, do any of you guys think that maybe we didn't break the chain? That maybe we were supposed to end up here?"
Just then, some construction workers accidentally knock over a part of the building they are working on, which causes a truck driver to swerve... and plow right in to the coffee shop they are all inside, killing all three of them instantly.