Following on immediately from the events of Rec 3, we are reunited with TV reporter Angela (Velasco), who was rescued from the infected apartment complex in which the series began, and taken to a cargo ship where Dr. Ricarte (Colome) is seeking a cure, using survivors such as Angela and Guzman (Manzanedo), a soldier who took part in the rescue mission to the apartment complex. However, one of their test subjects, an infected monkey, is released from its cage, and infects not just the vessel's cook, but also the food supply - the latter not being discovered until, of course, it's too late. Reviewing the last footage of Angela in the apartment building, Ricarte sees a parasitic creature infecting her, leading him to conclude Angela holds the key to finding an antidote, since she holds the original source of the outbreak. She has no memory of this whatsoever, and is understandably reluctant to let him operate on her, so it's probably fortunate that the horde pounding on the door lead to a delay in any surgical procedures.
I didn't enjoy this one as much as part three, which had a different director, the co-helmers of the first two parts having peeled off to do their own, separate installments. And having only seen the US remake of the original film, Quarantine, I can't say how the latest edition compares to its predecessors. He has, mercifully, abandoned the "found footage" conceit, and the closed-off system of the ship is clearly intended to provoke a claustrophobic feel. However, this requires more than its fair share of expository babble to explain, for instance, why no-one takes a lifeboat and bails out, and it turns out more restrictive, limiting things to a narrow set of rooms and gangways, which grows kinda dull after a while. There's also not much in the way of decent character motivation, beyond the obvious "survive to the next scene". You can see where Angela is coming from, but most of the others appear to act in ways that are more necessary to the plor, rather than making sense for their characters. On the other hand, there is one spectacularly silly sequence - involving a pack of monkeys and an outboard motor - which comes close to justifying the entire movie. Just, not quite.