Son Gokū

The is a tailed beast sealed within Rōshi of Iwagakure. It has been captured by Kisame Hoshigaki, then extracted and sealed by Akatsuki. It was later resealed into a resurrected Rōshi during the Fourth Shinobi World War.

Background
According to Tobi, the Four-Tails first came into being in the waning days of the Sage of the Six Paths' life in order to prevent the Ten-Tails' resurgence, the Sage split up its chakra and used his ability to create nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. The Four-Tails eventually fell into the possession of Iwagakure and was sealed into Rōshi at a young age.

Personality
From what has been seen of Son Goku, he detest being called by his generic name "Four-Tails". He is also a very prideful tailed beast boasting about who he was and his lineage calling himself "his holiness of all the heavens". He also seems to detest humans a great deal, partially because they have enslaved him. Like all apes however, he does tend to gibber.

Appearance
The Four-Tails primarily resembles a red-furred and green-skinned monkey, but with a muscular body of a gorilla. Its eyes are yellow and the pupils are white. It also has horns on its forehead, elongated canine teeth, and spike-like protrusions on its tails. The fur on its chin and on the top of its back resemble Rōshi's beard and hairstyle.

Abilities
The Four-Tails has the ability to simultaneously use the earth and fire natures, to spit lava from its mouth; something that hasn't been seen outside the realm of kekkei genkai. Since it is a tailed beast, it is able to use the Tailed Beast Ball. Four-Tails is also shown to be very strong as it was able to lift and toss the Eight-Tails.

Activity inside Rōshi
As the Four-Tails' jinchūriki, Rōshi could borrow its ability to create and use lava.

Itachi Pursuit Arc
After an unseen and difficult battle with Kisame, Rōshi was somehow defeated and captured by him. Kisame had difficulty capturing Rōshi because of the Four-Tails' power over lava. The Four-Tails was later extracted and sealed, leading to Rōshi's death.

Shinobi World War Arc
During the Fourth Shinobi World War, Tobi forces Rōshi into his full tailed beast transformation. With the Four-Tails on the battlefield, the great ape immediately attacks the Eight-Tails even managing to grab it by its tentacles and flip the beast over. It later attempts to seemingly swallow Naruto, but as the young jinchūriki attempts to free himself from between the ape's jaws, Son opens his mouth even wider and swallows him.

Now inside the beast, a chained up Son asks Naruto if he had come to steal his power as well, and lashes out at him when he refers to him as the "Four-Tails" telling him that he was the king of the apes and would be addressed with due respect. As their conversation continues, Son finds Naruto to be an open, honest person which shocked him somewhat. After cursing that all humans did was seal them away and deny their existence, Naruto tells him that he too was treated like that once and hated to see someone who did not know who they were controlling the beasts in this manner. After laughing at the prospect of Naruto wanting to be friends with his tailed beast, he realises that he is serious and tells the boy that there might be a way to stop him if the chains that bound him were undone.

Conception
The design of the Four-Tails is a combination of a gorilla and the Ōzaru from Dragon Ball. The horns on its forehead are modelled after the diadem worn by Sun Wukong from the classic Chinese novel Journey to the West, the character that Goku was based on.