There has been lots of advertising for this on Sky TV and the new date for Season 4 is Monday 7 April on Sky. They have also been repeating the first three seasons; if you don't have Sky the prices for the boxed sets of earlier seasons are coming down quite a lot. At present you can get all three of the earlier seasons for just under £45 if you look here, or they are available individually at prices ranging for about £15 to £25 (the first season of course is cheapest). The Season 4 DVD probably won't be out till next year, though.
If you are behind with the seasons or haven't seen the series at all, a boxed set is probably your best bet. In the meantime, it's getting quite hard to avoid spoilers as we keep seeing pictures of the cast on the internet etc - so at least you know who is still alive in Season 4!