My opinion about Season 4. I liked it but...

Ep01: Space Fleet. 9/10
The best episode of the series since Season 1. And I haven't seen one Star Trek episode. It was fantastic, really clever and full of fun moments and funny characters.

Ep02: Arkangel. 4/10
Boring and nothing new here. This premise was done thousand times in Twilight Zone, Outer Limits and even on The X-Files. It was also way too long for such a stupid plotline. (felt the same in many episodes from Season 3)

Ep03: Crocodile. 4/10
Same as Arkangel, it would've been way better if it was a short episode. (30 minutes tops) Don't know what's with these directors/writers/producers, if you can't pull off a good script for a 50 min episode, you have to shorten it.

Ep 04: Hang The DJ. 6/10
Good episode, but again, extremely long. The premise was interesting but I could see the ending after the first 10 minutes. This happens because of the lack of interesting characters and situations. Again, would've been a masterpiece if it was a 25 minutes episode (ala Twilight Zone), just tell a minimalist and sweet romance around the "system gimmick" and you're set for greatness. But no, we've got some drawn out and repetitive plot. Still, a good episode.

Ep05: Metalhead. 3/10
Interesting noir style story, BUT, it was by far the worst episode of the series, this was a short 5 min film stretched for over 40 minutes. And the robot was not menacing at all. B O R I N G.

Ep06: Black Museum. 9/10
I was a bored by this point. After Space Fleet, only Hang The DJ kept my attention, but this one? This one is a classic. Black Museum is pure genius and 100% good old story telling, this is the way to go for Black Mirror. This is what I was waiting for since the last couple seasons of The Twilight Zone and The Outer Limits back in 2002.
Three stories interlaced into one, and one big climax in the end, full of references from previous seasons and from other shows.
GREAT!

Overall:
Just for Space Fleet and Black Museum, this season deserves a watch. And because of those two episodes alone, it's way better than the boring Season 3.
I still can't understand why they keep trying with episodes longer than 40 minutes, they just can't maintain a good script for that long. More than half the episodes from Season 3 and Season 4 are stories that could've taken advantage of a shorter lenght episode.