The actress will play a con woman named Abigail, who blackmails Matt Bomer's protagonist Neal into joining her latest heist.
The character will also attempt to woo Diana (Marsha Thomason) as she works undercover to bring her in, according to TV Guide.
Mader, who played Charlotte Lewis in Lost, has also been cast in the final two episodes of Fringe next month.
House star Michael Weston and Smallville's Laura Vandervoort will also join the fourth season of White Collar.
The show has been confirmed to return to the USA Network on July 10.
Watch a trailer for White Collar season four below: