"USS Callister" was great but it felt like lesser Black Mirror to me. It was far, far, far too long and it basically just re-explored concepts from past episodes (most obviously White Christmas).
"Arkangel" was a decent premise but it's one of the worst episodes of the show to date. It just didn't really seem to know what to do with its plot after the initial premise was established. None of it played out in a particularly interesting way.
"Crocodile" was good but similarly lesser and just re-hashing old premises.
"Hang the DJ" was outstanding. The best of season 4 for me and one of the show's all-time best. I've heard it compared to "San Junipero" but I thought it was much better, personally. It was a good exploration of love and relationships with a decent sense of humour and I loved the ending. This episode also gave us one of the only new sci-fi concepts this season.
"Metalhead" was absolutely fantastic. I loved how simple and stripped down it all was. It was basically Black Mirror doing horror instead of sci-fi and I'm fine with that. Honestly, it's one of my favourites of the new batch - maybe of the show to date.
"Black Museum" was also excellent. The first segment was another horror story and I'd say the 2nd (and last) new sci-fi concept given to us this episode. The middle segment was absurdly close to that Karl Pilkington movie idea (to say that the first one was also something he's suggested) which took me out of it, but I still really enjoyed it - particularly the monkey toy aspect. The final segment was good. My only real complaint is that the first segment doesn't tie into the overall story like the last two both did. It's weird to have two connected stories and one that just feels completely disconnected like that.
Overall, I absolutely love Black Mirror and I hope it doesn't take too long for us to get more episodes. I can't say that I agree that it was better than season 3. There were more amazing episodes in this one than 3, but I'd say 3 was far more consistently fantastic if that makes sense. I also disagree that the second half was worse. After the first three episodes, I felt like this was setting out to be the worst season of the show to date and one that signalled Brooker was completely out of fresh ideas - one that suggests the show should end. The last three are what won me over. I loved them all. This season did feel like a departure from the previous three in that it's less about exploring sci-fi ideas and concepts and far more about characters and their emotional stories. I think that's likely down to Brooker running out of ideas, quite honestly. Still, if it leads to stuff like Metalhead then I'm fine with it.
And Danny:
"Hang The DJ" was a nice, more lighthearted story. I still don't quite get the ending, though?
The idea was that those two saw each other in a bar and used a dating app like Tinder but it's a mad sci-fi version of it that calculates how compatible you both are for a long-term relationship.
To calculate that, it runs simulations where it basically puts "cookie" versions of you both through the scenario of living in that world where you go through all these relationships and tears you apart from each other by saying you're a match with other people. If the cookies "rebel" against the system to be with each other, then they're considered a successful match.
The app runs the test 1,000 times in unison. We were watching test number 998 for the bulk of the episode. Once they all finished, they were added up which is how the app came to the number that the couple were a 99.8% match for each other.
The implication being that the real versions of those two will now approach in that bar and, being such an amazing match for each other, go on to be together forever after.