They ran out of rope on the original novels/BBC TV series at the end of Season 3, really, and I'm very curious to see how they're going to continue to stretch it. They really only have two major events from the original story left to depict. Then again, they've done a lot of stretching already, of course, as the BBC series only 3 series, each made up of 3 1h 45m episodes (each group of three representing one of the original novels) - so only 9 episodes total. The BBC series deviated quite significantly from the original novels, but the US version has followed the BBC one quite closely on a broad beat-for-beat basis.