Four-Wheelers: No wonder actors TONY ABATEMARCOand JOHN FLECK have been losing weight during their run of Charles Ludlam's fun-tabulous farce The Mystery of Irma Vep at the Tiffany Theater. The two, each of whom plays multiple parts, give "quick-change artist" a new definition as they dash offstage as one character and appear moments later as another. We shed a few pounds just watching (if only it were that easy!). As we were leaving the Tiffany, we couldn't help but notice the garish lights of Mel's drive-in, at the site of the former Ben Frank's, blazing up the block. And we got depressed thinking about the po-mo irony of an ersatz diner replacing the real deal. Actually, we always get depressed when the phrase "po-mo irony" pops into our head.