"The Four Horsemen" is an entertaining episode, with lots of interesting situations, a little bit of plot development and great moments for some characters that, however, doesn't seem to know where it is going.
Bellamy and Clarke team up with Jaha to search for salvation. Raven is losing her patience and in Arkadia things are getting worse and worse. Octavia tries to help the new king Roan. And the radiation may even be closer than expected.
All story-lines of the episode are well developed and some moments are surprising because they take some unexpected turns. It is helped with good acting and great mood, with the characters having to take some difficult decisions. Unfortunately, all the good aspects that keep making "The 100" great apocalyptic sci-fi cannot overcome the feeling that this new threat (right now) is not very mysterious or clear, in part because the plot doesn't seem to really know where it is going (maybe the writers stretching a little bit the main story-line). Nevertheless, "The 100" keeps being thrilling.