The foursome will all play new interns at County General Hospital. Appleby and Morris have been cast as siblings Daria and Andrew Wade. Rose will play the controlling Tracy Martin, while Rasuk has signed on to play Ryan Sanchez, a nervous New Yorker.
ER executive producer David Zabel told The Hollywood Reporter that the upcoming series will follow the four new faces as well as revisit the show's 14-year history.
"The new group of young characters - played by four charming and singular actors - adds a burst of fresh energy and a bunch of different personalities for us to play off," he said.
"So while we will be reconnecting with the history of ER, as well as telling the ongoing stories of our current regulars, we also will track the development of these new young doctors."
The fifteenth and final season of ER will air this autumn on NBC.