Naughty Dog has recently released an art book and inside is a variety of never before seen images. One of the most enticing images was some concept art for Jack and Daxter 4. This unfortunately means the game was in development at some point but it was scrapped before the completion of the game. The difference between this lost installment and the previous games is that the game seems alot more dark and violent. That may have been one of the reasons the development was scrapped.
Below you will see the images.