Tim Gunn finally squeezed the trigger. After negotiating with Bravo for months, the style guru has signed to return as mentor to the wannabe designers on the fourth season of Project Runway, starting this summer. Gunn, who's leaving his longtime job at Parsons School of Design to join Liz Claiborne, says in a statement, "I am thrilled to make it official that I will be back in the workroom. We are in the midst of casting the designers now, and I can already see that it will be another amazing season." But just to be on the safe side, Gunn is developing another show - Tim Gunn's Guide to Style, for Bravo. - Reporting by Ileane Rudolph