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.

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.

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.