As for Fringe they could very well cancel it, but I guess that giving it a season or half season to wrap up could be more likely, unless they've already asked its creators to wrap it up for the end of season 4... unless it's losing them too much money, or they need its slot for a show they hope will perform really better. There's not nearly as much to gain from pulling the plug on an unfinished series like Fringe after 4 seasons without giving it an ending as there is pulling a new series off the air. Pulling the plug on Fringe now means the DVD sales will become marginal, it will be difficult to sell it to any new buyer as the series is unfinished, and they'd lose the present revenues from foreign sales (typically, series like Fringe abroad are grid fillers, so buyers usually purchase them to the end of the series. It's more demanding for Fox to sell a new show to its foreign buyers than to renew a deal for one more season of Fringe...). That played in the decision to finish a show like Alias (not Fox, I know) despite its decline for instance.