Pacific Rim

Earth is under attack by gigantic alien monsters (called Kaiju) emerging from an inter-dimensional portal called "The Breach" at the bottom of the Pacific Ocean. Humanity unites to construct colossal robots, nicknamed Jaegers, to combat the alien threat. Each Jaeger is controlled by two or more pilots, who are mentally linked via a process called "drifting" to share the mental stress of piloting the machine.
In 2020, Gipsy Danger, piloted by brothers Raleigh and Yancy Becket, is assigned to protect the Alaskan coastline from a Kaiju, code named Knifehead. The monster survives the initial assault and blindsides Gipsy Danger, then rips open her hull and kills Yancy. Raleigh takes control of Gipsy alone and kills Knifehead. Traumatized by the loss of his brother, Raleigh quits the Jaeger program.
Five years later, Kaiju attacks have increased in both frequency and strength, to the point that Jaegers are being destroyed faster than they can be built. World leaders decide to cut funding for the Jaeger program in favor of anti-Kaiju coastal walls. Marshal Stacker Pentecost, veteran Jaeger pilot and commanding officer of the program, summons the surviving Jaegers and pilots to a base in Hong Kong called Shatterdome. He finds Raleigh and convinces him to rejoin the program and make a last stand.
At Shatterdome, Raleigh and Pentecost are greeted by Mako Mori, the director of the Jaeger refurbishment. He sees the remaining Jaegers and their crews: Striker Eureka (Australian), Cherno Alpha (Russian), and Crimson Typhoon (Chinese). The fourth Jaeger is Gipsy Danger, rebuilt and refurbished with additional weapons and nuclear reactor. Pentecost tells Raleigh his plan is to use the four Jaegers to seal the Breach with a nuclear weapon. To find a new partner to pilot Gipsy, Raleigh holds a tryout with the base's fighters, and finds his most "drift-compatible" partner to be Mori, who is revealed to be Pentecost's adopted daughter. In their test drift, Raleigh goes out of alignment after reliving the moment his brother died, causing Mako to become lost in her memory of the Kaiju attack that orphaned her, and nearly discharges Gipsy Danger's plasma caster. Pentecost grounds Mako, leaving Raleigh without a copilot.
Pentecost consults with Kaiju scientists Newton Geiszler and Hermann Gottlieb. Herman believes that the Breach is stabilizing, allowing increasing numbers of Kaiju to invade at a time, but allowing them to drop the weapon into the Breach. Newton, for his part, creates a device that allows him to drift with a Kaiju's brain fragment, and discovers that the Kaiju are bioweapons created by extraterrestrial colonists to wipe out humanity and invade Earth. Pentecost instructs Newton to find black market dealer Hannibal Chau to obtain an intact Kaiju brain and acquire more information. At Hannibal's place, Newton realizes that during his previous drift, the Kaiju's hive mind gained access to his knowledge, just as he gained theirs. Two Kaiju, code named Otachi and Leatherback, are unleashed upon Hong Kong simultaneously to find him.
All Jaegers, except for Gipsy Danger, are dispatched. The Kaiju easily destroy Crimson Typhoon and Cherno Alpha, and immobilize Striker Eureka with an EMP blast. Out of options, Pentecost allows Raleigh and Mako to pilot Gipsy Danger. They kill Leatherback, then pursue Otachi, which is hunting for Newton. After a lengthy and destructive battle, Raleigh and Mako manage to kill Otachi. Newton and Hannibal rush to Otachi's carcass, and discover that it is pregnant. The infant Kaiju bursts out and swallows Hannibal, before being asphyxiated by its own umbilical cord. Newton and Hermann drift with the infant Kaiju's brain, and learn that the Breach only opens in the presence of Kaiju DNA.
Pentecost reveals to Raleigh he is terminally ill due to piloting a first generation Jaeger with no radiation shielding. The last time he piloted a Jaeger was during the Tokyo attack, during which he was forced to kill the Kaiju that orphaned Mako alone, making him and Raleigh the only pilots who have killed a Kaiju without a copilot (known as solo combat). He has the remaining Jaegers proceed with the mission. With pilot Hercules Hansen injured during the previous fight, Pentecost and Chuck Hansen pilot Striker Eureka and carry the bomb; Raleigh and Mako provide cover in Gipsy Danger. At the bottom of the Pacific Ocean, the two Jaegers are faced with two Kaiju, code named Scunner and Raiju. Raleigh and Mako manage to kill Raiju and injure Scunner, but Gipsy Danger is crippled by the Kaiju. A third Kaiju, true to Gottlieb's prediction, emerges from the Breach. The Kaiju, code named Slattern, is a Category 5 Kaiju and the largest ever encountered. Slattern critically damages Striker, but they manage to injure the Kaiju. Pentecost and Chuck detonate the nuclear bomb, killing Scunner and further injuring Slattern. Considering Newton's discovery, Raleigh and Mako defeat Slattern and use the corpse to drive Gipsy Danger into the Breach to seal it with the Jaeger's nuclear reactor. Seeing that Mako has passed out from her oxygen supply being depleted, Raleigh gives Mako his supply and ejects her via an escape pod. He then manually overloads the reactor, just as it descends into the aliens' homeworld, and ejects himself back through the Breach.
Gipsy's reactor explodes, killing the aliens and sealing the Breach. Mako and Raleigh's pods surface in the Pacific Ocean, and the two embrace as rescue helicopters arrive. In a mid-credits scene, Hannibal cuts his way out of the dead infant Kaiju.

Question: Who is The fourth Jaeger?
Answer: Gipsy Danger