It shouldn't be surprising that "Half Baked," about four irascible pot-heads, opens with a flashback, though it happens to be one of the non-drug-induced variety. We are introduced to our quarter-bag quartet as adolescents smoking marijuana, getting wasted and having hallucinations for the first time. Pathetically enough, they are now adults, but are still living together in the same maturity-stunted state. When funds allow, the low-incomed underachievers ("The Nutty Professor's" Dave Chappelle, "Girls Town's" Guillermo Diaz, "RocketMan's" Harland Williams, and "Saturday Night Live's" Jim Breuer), indulge in illicit substances using their four-tubed ultimate bong (nicknamed Billy Bong Thornton). But when one, during a munchie-bloated stupor, unknowingly feeds junk food to a diabetic NYPD patrol horse and the horse dies, he's arrested as a cop killer. The others decide to sell stolen marijuana from a research lab to raise the $100,000 bail to save their friend.
   This joint venture manages to produce a few chuckles, but despite the comedic talents of its topliners, the film's low-brow humor and cartoonish antics send its boxoffice potential up in smoke. Just say no to this one. Starring Dave Chappelle, Jim Breuer, Guillermo Diaz and Harland Williams. Directed by Tamra Davis. Written by Neal Brennan and Dave Chappelle. Produced by Robert Simonds. A Universal release. Comedy. Rated R for pervasive drug content, language, nudity and sexual material. Running time: 82 min.