The big takeaway from the trailer for Paranormal Activity 4: the series has moved to the 'present day' (2011, at least) and maybe is following the baby kidnapped at the end of Paranormal Activity 2. That's just some guesses, though, as this series has been very happy to use stuff in trailers that never appears in the actual movies.
What this trailer does use, though, is a gag very similar to one of the segments in the upcoming horror found footage anthology V/H/S. The gag centers around a girl using Skype to talk to her boyfriend; on some level this seems like a gimme considering the genre. What makes this similar to V/H/S is the fact that both Skype gags center around spooky kids and slamming doors. If found footage continues as a genre then the Skype concept will eventually become de rigeur, but in the meantime it's important to keep in mind who was there first.
Another quick takeaway from this trailer: is the Paranormal series finally simply catering to tweens? The main girl in this looks to be the right age to know what Nickelodeon's line-up is these days. It would be interesting if the series tried to move to PG-13.