While the individual films may not do the greatest job of explaining things, if you step back and look at the story of the Paranormal Activity franchise as a whole, it's starting to become fairly clear: witches promised a female members of a certain bloodline to a demon who would collect the girls' first male heir as payment for... something or other.
Chronologically speaking, Paranormal Activity 4 is set in 2011, five years after the events of PA 1& 2 - twenty-four years after the events of PA3 - and we now follow a young girl named Alice (Kathryn Newton) who begins to document the creepy things that have been happening in and around her house, ever since a woman and her strange little boy moved into the area. When the young boy (named "Robbie") is taken in by Alice's mother, it starts a series of paranormal events connected to a dark past of demonic hauntings.
PA4's tagline - "All of the activity has led to this..." - would seem to be accurate, as we see in this trailer that everything from the fate of Hunter - Kristi's son from Paranormal Activity 2 - to evidence of the cult that sold young Kristi and Katie to a demon at the end of Paranormal Activity 3. It seems as though with this latest intstallment - which brings back PA3 directors Henry Joost and Ariel Schulman.
There also seems to be a larger cast of characters than indicated before, with Alice sharing screen time with her boyfriend, mother, little brother, and creepy houseguest, little Robbie (aka Hunter?). As per usual with the franchise, we get the an array of home video setups and set pieces - this time upgraded to the more modern age of web cams, HD recording, better night vision effects and generally better methods of constructing the found-footage format that made Paranormal Activity famous.
Whether or not this fourth film will wrap things up for good, or simply propel us down a new avenue of demonic terror, remains to be seen. As always, the answer likely resides with the box office returns.
Paranormal Activity 4 will be in theaters on October 19, 2012.