A fourth game was planned (it was in design document phase), but it never saw the light of day due to concentration on Myst 3 and the closure of Presto in 2002. With JP being an older game, I highly doubt there's any plans to resurrect the series. There is/was a fan-remake, but I can't make heads or tails whether they're still working on it.

Wait so no one from the old Presto team worked Pegasus Prime remastered?

Says who?
http://www.adventureclassicgaming.com/index.php/site/features/772/
http://www.youtube.com/watch?v=sjozUSCDOEw

I thought I recognized the faces in the credits! So, is there talk of JP4?

I've read that the document exists somewhere. If you like any other series like Broken Sword, Tex Murphy, or The Longest Journey, look at what they've been able to do with crowdfunding! I pray for the day that any of the authors (and I would be at the head of the line of people volunteering to do the research or write out the puzzles or dialogue) says, "Hey, guys, look at what these other series did on Kickstarter! The story's written, let's give it a try!"

What was previously Access (and was bought out by MS) was forced to leave the Tex Murphy game Overseer as a cliffhanger because they wanted to move in other directions. Chris Jones, who plays Tex onscreen, is a local, and---well, it took him a few years, but finally, courtesy of Kickstarter, the Tex Murphy saga continues (you have to get that one via Steam, I think---sorry, GOG!) I think if the Presto team could put their heads together and just get the word out that they want to make J4 a reality, they'd have the backing of their fans overnight. I'm so glad to see the voices of the fans showing corporate America how little it knows about what well-read fans want, and I'm thrilled to see an alternative business model that shows these buffoons that a good game isn't about brilliant 3-D graphics and how well one player can shoot everything in sight but can and should be an art form.

Now if I can just make the same sales pitch to Jane Jensen, whose Gabriel Knight series also got shelved as a cliffhanger...

Think again.