A fourth film in the Sleepaway Camp series was being made in the early 90s called Sleepaway Camp IV: The Survivor.
Production and filming had to come to a stop due to the company producing it going bankrupt. Only 34 minutes of footage was filmed.
On the box set "Sleepaway Camp: Survival Kit" the footage from the unfinished film was made available on it.
A fourth sequel did eventually come years later in 2008 called Return to Sleepaway Camp.