I haven't seen the 4th series yet but wanted to comment.
The 1st 2 series were excellent but maybe too long. 8 episodes stretched them.
Series 3 is absolutely terrible. Way too much needless pscho babble about the meaning of life and death bullshit.
It got so bad that when the murderer started his interminable rants I fast forwarded thru them. Some of the scenes between him and the detective defy belief.