ONCE again the Young Playwrights Festival has come along to enliven the New York theater scene. Fresh voices speak from the stage of Playwrights Horizons in a quartet of pieces as diverse in mood as they are theatrically stimulating. The four plays in this 10th-anniversary festival were chosen from 725 entries across the country by writers 19 and under. The winners and their ages (at the time the scripts were submitted) are Denise Maher (17), David E. Rodriguez (18), Matthew Peterson (17), and Carlota Zimmerman (17). Ms. Maher's "Secrets to Square Dancing" centers around young Karl, who confounds his elders with his slightly eccentric method of responding to one of those tests so dear to the hearts of trendy educators. Karl's behavior is considered serious enough to demand the attention of a parent-teacher group presided over by a person who employs phrases like, "All of us here are experiencing denial." Grown-ups would do well to be careful of what they say in front of youngsters - particularly young playwrights. Ka rl, who learns his own lesson, tells his mother, an affectionate single parent, "I better learn the right way to square dance so I'm less conspicuous." Mr. Rodriguez's "I'm Not Stupid," studies an unstable mother's behavior toward her retarded son, Roger. Roger wanted a hammer to build a clubhouse like the one put up by the Little Rascals. How his mother handled the situation is brought out in the course of her sessions with a psychiatrist assigned to investigate the troubled family situation. The mother defines her love for Roger as "the kind of love you have for a pet." Rodriguez probes a brand of maternalism that leads to bizarre, fatal consequences. With "Donut World," Mr. Peterson proves for the second time in the evening a gift for comedy among the young playwrights. Surrounded by his model railroad (one of Allen Moyer's more intriguing sets), Bud combines an encyclopedic knowledge of railway lore with a particular fondness for bakery doughnuts. He informs the spectator: "This railroad is my empire. It's a perfect world - at least as perfect as I can make it." Bud's ire is naturally aroused when his perky wife innocently refers to his tabletop emp ire as "toy trains." Here as elsewhere, the young playwrights comment pointedly on adult attitudes. By far the fiercest of the present prize winners is Ms. Zimmerman's ironically titled "Man at His Best." Written with a realism that deletes no expletives, the intense, highly charged drama concerns two prison inmates: Dean, a black hustler, and Skyler, his white cellmate. Within the forbidding, iron-barred cubicle of a cell the two men play games, taunt each other, and periodically break into violence. "Man at His Best" reflects an observation of criminal behavior that is not derived from watching TV cr ime dramas. In fact, the evening's finale shares the consistent denominator of maturity that marks these plays. As usual, the sponsoring Foundation of the Dramatists Guild has seen to it that the young playwrights receive first-rate professional productions. The variously directed casts respond with performances that enhance the values of the scripts. The actors include Louis Falk, Anne Lange, Curtis McClarin, Peter Francis James, S. Epatha Merkerson, Paul Bates, Olga Merediz, Seth Gilliam, and James G. Macdonald. The productions benefit visually from Elsa Ward's costumes, and Pat Dignan's lighting. The texts of two groups of previous prize winners are now available in Dell Laurel-Leaf editions: "Sparks in the Park" and other plays from the 1987 and 1988 festivals and "Hey Little Walter" and other plays from the 1989 and 1990 festivals.