La Monte Young, Terry Riley, Steve Reich, and Philip Glass are the four horsemen of American minimalism. Three of them will be active in New York in the next few days. On Saturday night, at La Monte Young's Dream House downtown, a group of string players will give an exceedingly rare performance of Young's hour-long Trio for Strings, which is often considered the first minimalist work — a 1958 piece in which notes are sustained for gaspingly long periods of time. Anyone expecting something comfortably tonal and hypnotic is in for a shock; Young was still under the spell of Webern when he wrote this, and the harmonies are harsh. But a new kind of musical space opens up: America. The trio will be performed in a new version, for sextet, in just intonation tuning. The presenters say, "We recommend that you wear light clothing." I. e., bring extra oxygen. Then, on Tuesday, the Film-Makers' Cooperative will give a benefit at Angel Orensanz, at which both Reich and Glass will be performing. Relations between the two masters of repetitive music have been cool over the last couple of decades, to say the least, so it will be a bit thrilling to see them in the same room. The concert also features Elliott Sharp, Todd Reynolds, Mark Stewart, Sue Garner, Patrick Watson, and a trio of Tim Barnes, Alan Licht and Lee Ranaldo, together with short films by Ken Jacobs, Michael Snow, Harry Smith, Jenn Reeves, Donna Cameron, Emily Hubley, Ron Rice, and Bill Morrison. As for Riley, his seventieth birthday will be celebrated at UCLA on Oct. 1, in a concert featuring Matmos, Acid Mothers Temple, pianists Joseph Kubera and Sarah Cahill, and the maestro himself.