With the disclaimer that I've not been seeking out theories (so when I say I haven't seen this theory in the wild, I mean in the three or four places I've seen the show discussed or reviewed), I'm leaning towards the fourth man being not a man at all, but rather it being Kate. There are a couple of reasons behind this (a combination of her former job putting her one step ahead of any anti-corruption investigation, her current job planting her right in the middle of the unit at the heart of the current anti-corruption operation, and what went down with her and Jo in the last episode, as well as in the moments the audience didn't witness between the previous two episodes), but the main one is that the show is clearly winding down, and setting things up so that none of the main trio will be left at AC-12 — and Kate being the head of a network of corrupt police officers would fit with that.

As I say, I have no idea if this is a popular theory, and I have a track record of getting these kinds of things wrong. But if I'm right, it fits with the bleak cynicism of the showrunner (who is clearly using this current series as a way to work through his anger at Brexit and the Johnson government).