I don't often get worked up about a CSO program, but then it's a rare thing for the orchestra to be performing something as adventurous and deliciously ludicrous as the Ives Fourth Symphony. It requires no less than two conductors, a full-scale symphony orchestra, a chorus, a brass band, three pianos (including one that's been detuned a quarter tone), a celesta, an organ, timpani, Indian drums, gongs, a glockenspiel, a Theremin (an early synthesizer), and a separate celestial choir of five violins and two harps. The stage at Orchestra Hall is being extended over the first three rows of seats to make room for it all. Replete with quotations from American hymns, folk tunes, patriotic songs, and marches, the second movement is a uniquely Ivesian account of the Fourth of July at Concord, with competing bands and drum-and-bugle corps. The last time the CSO did this piece was more than a decade ago, but this is a special occasion--a warm-up for the CBS recording sessions that will follow. Conductor Michael Tilson Thomas (who will be assisted by James Sinclair) is a master Ives interpreter who gave us that spectacular Holiday symphony three seasons ago. As an opener, Emanuel Ax will play the Brahms Second Piano Concerto, and members of the Chicago Symphony Chorus will sing some of the songs used in the Ives piece. It should be spectacular. Tonight, 8 PM, Sunday, 3 PM, and Tuesday, 7:30 PM, Orchestra Hall, 220 S. Michigan; 435-6666 or 435-8122.