I suppose since the BBC lost the UK terrestrial TV rights to the show, and Fox/Sky priced every single terrestrial broadcaster out of the rights to series 3, forcing non-subscribers to rent or buy the DVDs, they're stuck on their own screening series 4. Serves them right.
Oh, and last year I dared Fox to sue me for downloading series 3. I'm doing the same for series 4, even though I'm now only six weeks ahead of seeing it legitimately if this simulcasting arrangement continues for the rest of the series.
Edit 21:03: It's a one-off, as csogilvie thought. With a SKY TRAVEL watermark on it. Bastards.