If you're considering buying this, then I guess you've seen season 3. The second half of season 3 was quite poor, so if you stuck through it, then this is your reward!
Season 4 starts out in the same vein as season 3 ended, which is to say, a bit ropey. After about ep4 though, the writers really hit their stride and from then on in, the procedural side of the show is very high quality. Episodes 6-8 are absolutely top-notch TV and probably worth the price of the box set alone.
There are a couple of new recurring characters, which helps mix things up a bit, though outside of that there is surprisingly little character development; the one subplot there is is tied up too quickly and neatly, and could have run for the whole season.
Finally, unlike previous seasons, this one does not really go out with a bang or incredible cliffhanger, which was a bit of a disappointment.
Still, I was considering giving up on WAT after season 3, and I'm really glad I got season 4 - and I'll be buying the next season as soon as it's available, too.