Flashing back to the end of part 3, it seems that Yancy Butler's character Reba wasn't killed after all (despite the fact she was clearly seen bitten in two) and is now working for the Dept of Wildlife, who along with new Sheriff (Elisabeth Rhom), is helping contain and study the giant crocs in the lake.
But when a group of poachers, led by a chap named Jim Bickerman (presumably some relative to the Bickerman's in the previous films, played by horror veteran Robert Englund), break into the compound to bag themselves some trophies, they leave the gate open and a bunch of teens, planning a lakeside party, inadvertently blunder into the reserve.
More mayhem quickly ensues, as the CGI crocs begin to chow down on yet more teenage victims, who are partying along the lakeside shore, whilst Reba, the Sheriff and her deputies try to fight their way through the croc infested compound to rescue the survivors.
This is yet another made-for-TV sequel by the Sci-Fi channel (or SyFy as they like to call themselves now), which basically serves up more of the same, whilst aping the basic plot to "Jurassic Park", but with crocs instead of dinosaurs. This one is a slight notch above the other 2, thanks to the inclusion of Robert Englund, and like the other sequels is watchable, but is otherwise fairly unoriginal and only for those who love these tacky, badly done CGI animated horror films.