I wouldn't mind seeing a 4th JP movie. I loved the first one. I really enjoyed the second one, although not as much as the first. The third one was okay, but not nearly on par with either of the first two.
For casting/character? Bring back all the surviving characters from the first movie, and hopefully the actors who portrayed them as well (with the kids, you can probably get away with recasting them if the original actors don't want to do the film). Then add some fresh new characters. Some of them to be dino-fodder, others to be recurring characters in any future JP films.
As for plot? That's a tough one. I've read some interesting ideas on this thread. I kinda like that biodome/zoo idea. Or some plot that sends the original characters back to the original islands. I would say a rescue mission, but that's been done.
The Dawn Of The Dead idea isn't bad either. Maybe the dinosaurs break free from the lab/zoo, possibly by some animal rights eco terrorists. A team of heavely armed big game hunters and dinosaur experts are sent out to either recapture them or eliminate them. But they're breeding far more quickly than anticipated, and have been genetically altered to grow to maturity within days. Thus it becomes less of a case of just rounding them up and more of a case of multiplication. Like tribbles with teeth.