Voice of Elmo Kevin Clash -- Sued by 4TH Accuser

Kevin Clash has been sued by a 4TH accuser ... who claims he also had a sexual relationship with the former voice of Elmo while he was a minor ... TMZ has learned.

We've learned ... the accuser, who filed the lawsuit in federal court in NY under the name John Doe, claims he first met Clash in Miami Beach in 1995 or 1996 ... while Doe was 16 or 17 and Clash was in his mid '30s.

Doe claims ... Clash eventually flew him out to NYC and gave him money and a place to stay. During the trip, Doe claims the two had sex on multiple occasions.

According to the lawsuit, obtained by TMZ, Doe claims he's suffered tremendous emotional, mental and psychological damage as a result of the relationship.

TMZ spoke to Doe's attorney, Jeff Herman, who tells us, "According to our lawsuit, Kevin Clash knowingly paid to transport a minor across state lines for the purpose of satisfying his sexual interests."

Of Clash's 4 accusers, only 3 have filed lawsuits. The first accuser struck a settlement with Clash and agreed not to pursue litigation.