At the recent London Expo, there was a Primeval panel, at which some spoilers for Series 4 and 5 were talked about, among other things.
fredbassett posted a lovely bit of material over on the primeval_itv community about stuff, but be warned, there's a lot of spoilers over there.
One of the elements of Primeval that the show changed direction and/or got away from was the anomalies themselves. Sure, while Nick Cutter and his crew (and later the ARC) had to deal with the incursions, there wasn't a lot dealt with in terms of what the Anomalies are, what their origins are, and that sort of thing. One of the things I'm glad about is the new series of the show will get back to some of these questions about the Anomalies.
I like the idea that some of the characters from the first three series of the show will be re-appearing, even if there are no solid hints about who, other than the fact that Jenny Lewis will be back. I'm also glad that James Lester will be in each episode, since Ben Miller is just great in the role, and gets a lot of really witty, snappy lines. I keep entertaining hopes that Douglas Henshall will return to the series for an episode or three, but doubt that's going to happen, but if Helen can re-appear, why can't Nick? For that matter, why not Claudia Brown? I really do miss Nick Cutter.
I'm looking forward to seeing new beasties and creatures in the series, but have to say that I'm rather looking forward to more DINOSAURS. While I find future creatures and predators fascinating, I have to admit that I want to see more dinos! That and some human villains and the like along the lines of some of what we saw with Oliver Leek and Christine Johnson.
Overall, I can't wait to see Primeval series 4 and 5.