I've just replaced sensor 3, but this was after the DPF and before the flexi joint and cat, sensor 4 seems to be the one after the cat. A rescan showed that sensor 3 now has a realistic temperature at cold, so this was definitely one of the problems. I've just got to see if I can get to sensor 4 on my own, otherwise it'll have to go to the garage and up on a ramp.
For info, sensor 3 giving a reading of 999C when cold, measured as open circuit.