Everything I've already said about the series continues to apply: it's very well constructed, it feels realistic, it's a pleasure to watch, it's frequently dark.
As an example of what I love about the writing: Da Vinci has been driving the same SUV since the beginning of the series. As a viewer, you don't really think about this - but that means the car is a minimum of five years old, possibly older. In the last episode of this season, he's headed for a cemetery for a disinterment when the car sputters and stops, so he's desperately calling his office and then walking to the cemetery, where he's half an hour late. It's not a big or important scene, but it clearly shows the difference between this show and every other "cop show."
Mick Leary (Ian Tracey) is having more and more trouble holding his life together as the after-effects of something that happened mid-way through season 4 are tearing him apart. Angela Cosmo (Venus Terzo) is trying (only semi-successfully) to befriend a young prostitute. Which leads her to a couple cops in Vice: Suki (Camille Sullivan) and Brian (Colin Cunningham). Across multiple episodes, we find out that Brian is severely bent, but not quite bent enough for them to pin anything on him. And not surprisingly, Suki wants desperately to not be Brian's partner.
The show is never-ending. And yet in most episodes they manage to get a reasonable wrap on a case or two, giving us some sense of satisfaction.