Thiruthanikan Thanigasalam, 40, and Sahilal Sabaratnam, 29, pleaded guilty in Brooklyn, New York to conspiring to purchase guided anti-aircraft missiles for the Liberation Tigers of Tamil Eelam (LTTE), which the US designates as a foreign terrorist organization.
On Monday, co-defendants Sathajhan Sarachandran, 29, and Nadarasa Yogarasa, 54, pleaded guilty to similar charges.
Thanigasalam, Sabaratnam and Sarachandran face a 25-year sentence to life in prison, while Nadarasa faces up to 30 years prison.
The four defendants were arrested on Long Island, New York in August 2006 after three from the group were accused of negotiating with an undercover FBI agent to buy and export military equipment for the Tamil Tigers.
The equipment included heat-seeking anti-aircraft missiles and launchers, and 500 AK-47 assault rifles.
The United States considers the LTTE a terrorist organization.
Sri Lankan troops captured the last Tamil Tiger stronghold of Mullaittivu on Sunday, wh