This was, by far, my favorite episode of season 4. In general I found season four to be heavy-handed and emotionally manipulative in a way that, while not out of character given the previous seasons, is increasingly annoying. There were a couple exceptions, and Metalhead was one.
The story takes place in a post-apocalyptic countryside that appears to be largely devoid of people. With beautiful black and white cinematography we follow a trio of scavengers as they run afoul of an apparently autonomous guard robot which has the ability to stalk the intruders. The machine, referred to as a "dog", is terrifying, adept at its job, and not unlike robots which are currently under development.
Some reviews criticize the lack of context, but I think that the minimalist approach is perfect. The episode asks more questions than it answers, but they are all questions worth asking; What are these things? Who put them there? Were they left there or did they come from somewhere else on their own? How long ago? Is it smart to make autonomous intelligent machines which are capable of murder?
Black mirror got this one right.