Line of Duty: Series 4
* * * - Okay
It's clear creator Jed Mercurio, who's written every episode of the series, didn't have as much time to come up with (what I assume will be) this new trilogy. It's still a good show, with the same feel as the previous seasons, but there are less twists and more unanswered questions. The tight plotting of Series 1-3 is replaced by viewer knowledge that questions will eventually be answered. So while Series 4 does resolve the main plot, there are a disappointing amount of unanswered questions, some of which are tied into all 4 seasons. I wish Mercurio had left Series 1-3 alone. There was no need to unwrap that bow with new possibilities.
I think my other reason for the lower rating is the focus on Thandie Newton, as the main officer under investigation. The past 3 seasons had very specific targets and actors made strong impressions by knocking their parts, big and small, out of the park. For Series 4, they lured in a bigger name but not really a better one. Newton is competent but not nearly as compelling as those who came before her.
I'm still in and will definitely be watching Series 5 when it drops, but for now I strictly (and highly) recommend Series 1-3 only.