I don't think it'll be too much of a shock if Jack bites it as many TW fans have been predicting his demise since Ep 1. It would be a stunner if Gwen was to die, simply because RTD has said how untouchable she is (and RTD has already commented in an interview that he has already got a plot for S5 which focuses on Gwen, implying Jack is either a bit player or not there at all). My money is on Jack, unfortunately, although I wouldn't put it past RTD to make Jack toss poor Anwen into the giant vagina as a sacrifice.