The class-action suit, filed Tuesday in New York, said the four defendants shared nonpublic information about client purchases and sale orders to manipulate prices for their benefit and to the detriment of plaintiffs.
This illegal sharing of information "gave them the ability to execute trades... in advance of those (price) movements," the complaint said.
"This unlawful behaviour allowed Defendants to reap substantial profits, while non-insiders, which include Plaintiffs and members of the Class, were injured."
The plaintiff, Modern Settings, a US maker of jewelry and other metal products, alleges it lost value on "tens of thousands of transactions" due to the conspiracy, the complaint said.
Investors in the commodities "lost millions of dollars as a result of this conduct," said Labaton Sucharow, the firm representing the plaintiffs, in a statement.
The complaint alleges the manipulation began as early as 2007 through the present. It seeks to bar the illegal conduct and gain unspecified financial damages.
The four defendants participated in twice-daily "fixings" teleconference calls to set prices for the physical metals markets in a process set up by the London Platinum and Palladium Market in 1987.