Clash, who provided the voice for iconic red monster Elmo for more than 20 years on "Sesame Street," recently resigned from the pre-school television program to focus on his legal troubles.
He has denied any wrongdoing in the three previous cases, but has not responded to requests for comment regarding the latest allegations, TMZ said Monday.
The fourth accuser filed the lawsuit against Clash, now 52, in federal court in Manhattan under the name John Doe.
In the legal papers, he said he met Clash in Miami Beach, Fla., in 1995 or 1996, while Doe was 16 or 17, then traveled to New York where Clash lives and had sex with him on multiple occasions, TMZ said.
This is the third lawsuit brought against Clash this year. The first accuser settled out of court and agreed not to pursue litigation, but has since expressed regret about the decision.