The filmmaker, who steered the third franchise instalment, told Ain't It Cool News that the Jurassic Park 4 script takes the series into new territory and sets up a new trilogy.
"There is a great story for the fourth one that I would be interested in getting involved with and it's nothing like the first three," he said. "It sort of takes the franchise off in a completely different direction, which is the only way I would want to get involved."
Discussing the setting for the fourth film, Johnston insisted that it would be pointless to bring the action back to the island: "Why would anybody go back to that island? It was hard enough to figure out the second and third reason for them to go, but [the new movie] would take [the series] off in a whole other trilogy basically."
An earlier script for Jurassic Park 4 allegedly had the government genetically recreating dinosaurs and training them to carry weapons.