"The Big Four" is a group formed by Gold, Silver, Bronze, and their fusion, Platinum. They are created by homeworld. Actually, they are results of a failed experiment. The homeworld gems tried to destroy all metals, but they managed to escape. The Big Four hate homeworld because of that.
Gold is a calm metal, like Garnet. His metal is his right eyeball. He can summon a bow and arrows.
Silver has a depressive personality. Her metal is on her upper arm. She can summon a mace.
Bronze is a joyful metal. His metal is his hand. He doesn't have a weapon, he has psychical powers.
Platinum has an angry personality. It has all three of the metals, its eye, arm, and hand. It can summon a weapon that can transform into any weapon.