The chances for season 4 are very good, especially with the strike. The CW was caught with its pants down. It has no scripted pilots available. Zero. Kripke says he expects a fourth season.
You see, CW is half owned by Warner Brothers, which is SPN's studio. Supernatural currently airs in 46 foreign markets. That's major money in the bank for Warner. They're going to want four or five seasons for a nice, 100 episode second run syndication deal now. Ackles admitted this outright at a con in London last May to the attendees. As long as Warner considers SPN a cash cow, it's safe. Dawn Ostroff wanted to cancel it after last season in favor of her new teen shows, (which have bombed in the live ratings) but the higher ups at Warner stepped in and said, "No, you won't."
Warner also produced Veronica Mars, but VM sold like crap in the overseas market. Ergo, it was cancelled.