Optimus Prime - Leader of the Autobots who has been upgraded and now transforms into a rusty Marmon cab-over truck and later a Western Star 4900 Custom. He now wields a sword and a shield.
Bumblebee - An Autobot scout. He has been upgraded. His new alternate mode is a modified 1967 Camaro SS and later a 2014 Camaro concept. He can transform both arms into plasma cannons, which are also missile launchers. In the trailers, Bumblebee can also be seen using a cable gun. He speaks using radio recordings from his alt-mode (although it has been shown he can speak normally).
Ratchet - An Autobot medical officer who transforms into a green and grey search and rescue Hummer H2. His weapons include a Cybertronian rotary saw that emerges from his left arm and can transform his right and left arm into a medigun. He is brutally killed by Lockdown and Cemetery Wind.
Leadfoot - An Autobot who is also member of the Wreckers. Transforms into a Red #42 Juan Pablo Montoya NASCAR Target Chevrolet Impala. He was killed by Cemetery Wind, as shown via recordings in the film.
Brains - A former Decepticon brain unit who had escaped and joined the Autobots. He transforms into a Lenovo ThinkPad Edge laptop. His right leg is damaged and he relies on a make-shift cane to move around. He was captured by KSI and forced to analyze the brains of various deceased Transformers to help build the human-made prototypes.
Hound - An Autobot commando who transforms into a Medium Tactical Vehicle from Oshkosh Defense. His robot mode has a beard, a "beer belly" similar to Leadfoot, and a bullet for a cigar, as well as numerous weapons, make him look like a stereotypical gung-ho soldier. He has a southern American accent.
Drift - A former Decepticon tactician who has joined the Autobots. Transforms into both a helicopter resembling Mil Mi-24 and a Bugatti Grand Sport Vitesse. His robot mode is similar to a samurai, and his helicopter blades from his alt-mode serve as his swords. He has a Japanese accent.
Crosshairs - An Autobot paratrooper who transforms into a C7 Corvette Stingray. His robot mode has a "trench coat" and goggles, making him look like a pilot from the 20th century. He also has a British accent.
Grimlock - Leader of the Dinobots who transforms into a metallic horned Tyrannosaurus Rex. He lacks the ability to speak and acts like primitive beast. He attacks Optimus Prime when asked to join forces but is soon tamed and allows Optimus to ride him into battle.
Strafe - A metallic twin-headed, twin-tailed Pteranodon.
Slug - A metallic spiked, bestial Triceratops.
Scorn - A metallic three-sailed Spinosaurus.
Decepticons who appeared in the last three movies have all been killed and Michael Bay stated that it will remain that way. Though he said that before becoming the director of the fourth movie and Bay has been known to change his mind. Megatron's remains, however, are seen in the film and they have a significant role regarding the human-made Transformers. StarScream's head is also seen when Joyce is giving a tour of his facility.
Comment: I have watched this several times and I cannot see Starscream's head anywhere. However, it appears that Sentinel's head is close to Megatron's in the lab. This does not bode well...
Galvatron - A human-made Transformer who becomes posessed by Megatron's mind, thus making him the new leader of the Decepticons. Transforms into a 2014 Argosy Freightliner.
Stinger - A human-made Transformer with a robot mode based on Bumblebee who becomes a Decepticon spy under Galvatron's control. Transforms into a 2013 Pagani Huayra.
Junkheap - A human-made Transformer who becomes a Decepticon under Galvatron's control. Transforms into a Chinese garbage truck of Waste Management Inc., and his robot mode is capable of splitting into three components. Replaced Wreck-Gar, who was originally scripted to appear.
Traxes - Numerous human-made Transformers who all become Decepticons under Galvatron's control. They transform into different colored (red, blue, black and silver) Chevrolet Trax cars.
Unknown human-made Transformer - Transforms into an orange McLaren MP4-12C. Robot mode never shown in the film.
Unknown human-made Transformer - Transforms into a white Aston Martin DBS. Robot mode never shown in the film.
NEW TRANSFORMER BOUNTY HUNTERS
The Transformer Bounty Hunters are explicitly explained in Michael Bay interviews, in film dialog, and in recent cartoon shows to be neither Autobot nor Decepticon, but a neutral third party that works for the alien creators of the Transformers. None the less their methods are clearly malevolent.
Lockdown - An intergalactic bounty hunter and the main antagonist. Transforms into a Lamborghini Aventador LP 700-4 Coupe. He is killed by Optimus Prime.
Steeljaws - Numerous techno-organic wolves working for Lockdown.
Numerous unknown humanoid robots without alternate modes working for Lockdown.
There are also various aliens that are found within Lockdown's ship, though they are captured and locked up in cages.