It is hard to believe that in 2006 nobody had ever heard of “Paranormal Activity.” The original came out in 2007 and exploded mostly via word of mouth. Now, in the last three years, like clockwork, a new “Paranormal Activity” comes out.

There are drawbacks to making a series of sequels to a movie. The elements that were so effective in the original grow old and tiresome as they are repeated in subsequent movies. And as more information is revealed about the characters and events, it can diminish the impact of what were the mysteries in the original.

At the shocking end of “Paranormal Activity,” one of the most unnerving aspects was that the viewers did not know what was haunting Katie and Micah, eventually possessing Katie. All we knew was that Micah was dead and Katie was missing. That’s an ending that sticks with you.

Inevitably, given the success of “PA,” three more sequels have hit the theaters. Yes, we have learned more about Katie, but are we better off?

So, let’s recap: We meet the cute couple Katie (Katie Featherston) and Micah (Micah Sloat), unmarried but living together in a nice home in Southern California in 2006. Katie is a student and Micah is a day trader. Because of strange happenings in the house, Micah invests in a video camera he sets up in their bedroom to perhaps record any paranormal events. Per usual, the activities — at first benign events, like doors moving or lights switching on an off — escalate, and Katie, now possessed, kills Micah and disappears.

In 2010, “PA2” came out. It really was a prequel, taking place months before the terror seized Katie and Micah. “PA2” focuses on Katie’s younger sister Kristi, who lives in Carlsbad. She is married to a man, previously married — likely widowed — who has a teenage daughter, Ali. They have just had a son, Hunter. When their house is found ransacked, they have security cameras placed throughout the home. Once again, little bumps in the night soon become more intense occurrences. Kristi talks to Katie about the incidents, leading Katie to refer to similar experiences they had as children and how it is best not to goad whatever this entity is by investigating further. Daughter Ali’s online probes indicate that the hauntings may be the result of an effort to seize Hunter to settle an old family debt by claiming a first-born son.

Kristi soon seems possessed and husband Daniel, at the advice of their superstitious nanny, conducts a ritual that transfers the haunting from Kristi to Katie — with disastrous results. Katie appears, having already murdered Micah. She kills her sister and brother-in-law, abducts baby Hunter and disappears.

In 2011, “PA3” came out, exploring the childhoods of Katie and Kristi. It is the 1980s and the two girls are living with their single mother Julie, but she has a live-in boyfriend, Dennis. Kristi has an invisible friend named Toby. When strange things start occurring, Dennis, with the help of his friend Randy, set up what was then bulkier video equipment to catch on tape any funny things going on. Hence, the usual increase in paranormal shenanigans, terrorizing to the point the family flees to the home of Julie’s mother. There it is evident that Grandma might know what is really going on. In an intense but confusing ending, Katie and Kristi are soon rendered orphans but passively latch on to their grandmother. And there are hints of a coven.

“PA3” left us with a few baffling moments and had us wondering how Katie and Kristi, not only seemingly well-adjusted despite the terrors they endured as children, also are baffled when these hauntings resume as they become adults. You would think they would be accustomed to it.

So, on to “PA4.” The trailers show a new character, a teen girl, who through webcam chats with a friend, talks about a strange little boy in the neighborhood. Then the trailers show the usual “PA” ingredients, including people being dragged away by some unseen force.

“PA4” takes place in November 2011, in Henderson, Nevada. The teen girl is Alex (Kathryn Newton), who lives with her parents Doug and Holly (real-life couple Stephen Dunham and Alexondra Lee) and a younger brother Wyatt (Aiden Lovekamp). As “PA4” begins, a youth soccer game is in progress in which Wyatt is participating. Briefly seen is a young boy on the other side of the field, alone, watching the game.

The lone boy is named Robbie (Brady Allen), who lives across the street from Alex’s family but often wanders over to sit in Alex’s old treehouse.

Alex spends most of her time on live webcam chats with Ben (Matt Shively), who also seems to have ample access to Alex’s home, day and night. Meanwhile, Alex is concerned because Doug and Holly seem to be drifting apart.

One evening something happens at Robbie’s house and his unseen mother is taken away by ambulance. Even though Alex’s parents barely know Robbie, they take him in, as he apparently has no other relatives. Robbie and Wyatt soon become friends but coinciding with Robbie’s presence in the house are paranormal events. Robbie seems to have emotional control over Wyatt, who soon starts talking to unseen entities.

So, does this all tie in with Katie and Kristi? Well, that’s really the only intrigue about “PA4.” The thumps, doors opening, swinging chandeliers — that’s old hat now.

One thing the “PA” movies excel in is having pathologically irritating male characters that began with Micah. In “PA4,” Doug gets a little screen time, being the usual skeptical adult, but it is Ben who is the designated goof, the techno expert who helps Alex set up webcams throughout the house while pulling off dumb pranks and putting clumsy moves on Alex.

Like its predecessors, “PA4” revs up the action in the final minutes, where the real jump-in-your-seat scares occur. Of course, it leaves us wide open for a fifth entry.

“Paranormal Activity 4” was written by Christopher Landon — son of Michael — who has taken up the writing chores of the “PA” franchise from Oren Peli, who wrote and directed the original — although it’s generous to say Peli wrote that script. He has admitted he wrote only guidelines and had the actors mostly improvise their dialogue.

“PA4” suffers from redundancy but at least it is making an effort to branch out a bit, which does soften the impact of the original story but adds substance to the creepy tale.

In a sad postscript, Stephen Dunham, who played Doug in “PA4,” died on Sept 14, his 48th birthday.