We know very little about Paranormal Activity 4's plot, but we do know who was hired to write the script. According to The Hollywood Reporter, The River writer/producer Zack Estrin has been picked up by Paramount to write the screenplay for the forth entry in the successful found footage franchise.
Estrin has spent many years writing for television on shows such as No Ordinary Family, Prison Break, and Charmed. With Paranormal Activity producer Jason Blum and creator Oren Peli both involved with The River, it makes sense that Paramount would hire someone that the producers are familiar with.
It was previously revealed that Paranormal Activity 3 directors Henry Joost and Ariel Schulman will return for the sequel, which has been scheduled for release on October 19th. Paranormal Activity 3 earned over $100 million in the US theaters on an estimated production budget of $5 million, so expect the franchise to continue as long as they have interesting stories to tell. Plot details for Paranormal Activity 3 were kept under wraps until only a few months before the movie's release, so we don't expect too much information to be revealed until later this year.