According to Heidi Klum's Website, Season Four will premiere on November 14th. Thanks Curtis.
ETA: Not surprisingly, the blurb on Heidi's website has disappeared! Here's what it said:
"Project Runway Season Premiere-November 14th
Watch the new season of Project Runway beginning Wednesday, November 14th, only on Bravo (check your local listings). I consider it to be our best season yet!"