Prosecutors charged Jon, 33, with another 15 felonies and five misdemeanor counts involving the additional four alleged sexual assault victims, including a 17-year-old, the Los Angeles Times said Tuesday.
The fashion designer was in Los Angeles to unveil a new line during L.A. Fashion Week when he was originally charged earlier this month.
"He lured these girls to be models and once here sexually assaulted them," Jane Robison, a Los Angeles County district attorney's office spokeswoman said when the new charges were filed Monday. "This is an ongoing investigation and we believe there may be numerous additional victims."
New York authorities also are investigating into whether Jon assaulted women there, Robison said.
Jon was charged March 13 with seven counts, including forcible rape, lewd acts upon a child, contributing to the delinquency of a minor, sexual battery by restraint and attempted forcible oral copulation. Those charges involved three alleged victims, including a 15-year-old.
Jon's attorney, Ronald Richards, said women sought to exploit their relationship with his client because of his celebrity status.