Now at its fourth installment, the “Paranormal Activity” franchise is running on fumes, but that didn’t stop the “Saw” movies, which it’s replaced as a Halloween standard, from dragging on interminably. So if this latest chapter scares up enough of an audience to show a profit—which shouldn’t be difficult, given that it looks as though it was made for next to nothing—one can expect another next year.

That despite the fact that the present flick continues the series’ downward spiral, being by far the weakest of the lot. Meandering dully for more than an hour, it rouses itself to deliver some sudden shocks in the last reel; but since none of them contribute to anything resembling a coherent story, they seem just a pointless attempt to generate some cheap last-minute excitement.

The plot, to use the term loosely, is mysterious in the worst sense—so full of holes and meaningless “gotcha” moments that it’s incapable of being parsed. It begins where “Paranormal 2” ended (the third film, of course, being a prequel), with possessed Katie (Katie Featherston) killing her sister Kristi and snatching her infant nephew Hunter. Five years later we meet nice suburban family consisting of dad Doug (Stephen Dunham), wife Holly (Alexondra Lee), teen daughter Alex (Kathryn Newton) and six-year old Wyatt (Aiden Lovekamp), who we are eventually told is adopted. Also a semi-permanent figure in the house is Alex’s snarky boyfriend Ben (Matt Shively).

Things heat up when the woman who’s just moved in across the street falls ill, and little boy Robbie (Brady Allen) comes to stay temporarily with the neighbors. He’s a strangely inactive kid with an imaginary friend and a propensity for wandering around at night and appearing suddenly when you don’t expect him to. But he and Wyatt hit it off and are soon engaging in some curious secret rituals of their own. But his arrival coincides with the start of the familiar poltergeist-like phenomena in the house, like bumps in the night and ominously closing doors and furniture than moves by itself.

What’s going on? Alex and Ben set up cameras to watch what’s happening—allowing for the customary grainy footage series fans have come to expect, complete with the predictable vague apparitions and sudden jumps. But even before that point, virtually the whole picture has been shot via camera-POV perspectives in jittery style. The apparent presumption is that Ben and Alex are constantly filming each other via their computers and cameraphones, and Alex, even when walking around her house alone, records everything with a camera we glimpse only very occasionally. Kids and their electronic gizmos nowadays!

When Robbie’s mother returns unexpectedly from the hospital, damned if it doesn’t turn out to be Katie. As it turns out, she has her sights on a member of the family, but which one, and why? And what’s Robbie got to do with it all? The answers to these questions are given, but in an elliptical fashion that doesn’t tell us much. And when Katie starts walking around like a robot—apparently the most demanding form of acting of which Featherston is capable—a goodly portion of the other characters suffer unpleasant fates. And the movie ends with a big climax—a series of them, really—that come out of nowhere and seem random rather than integral to what’s happening.

“Paranormal Activity 4” does have a few genuinely creepy moments—like a sequence with Wyatt in a bathtub that doesn’t make much sense but creates a sense of unease, though it does so rather despicably by putting a child in jeopardy. But for the most part it’s curiously tame and tedious. It certainly doesn’t help that the makers occasionally drop in homages to truly great horror films, because they call up comparisons that a mediocrity like this should really avoid. (A sequence in which Wyatt rides around on the same sort of tricycle Danny circled the Overlook on in “The Shining” only reminds us how much more unsettling Kubrick’s masterpiece remains.)

The acting down the line is utterly perfunctory, except for Featherston, who’s simply terrible, and kids Lovekamp and Allen, who at least have some personality. Newton and Shively could have stepped out of any teen horror movie you’ve ever seen, and Dunham and Lee are blander than bland. (Dunham died last September of a heart attack at age 48. This movie is a sad swansong.) On the technical side the picture’s okay, with cinematographer Doug Emmett playing up the obvious shocks, even if Gregory Plotkin’s editing often allows things to flag despite the short running-time. But that’s probably due to the pro-forma footage that directors Henry Joost and Ariel Schulman saddled him with. They seem to have run out of ideas here.

This latest “Paranormal” effort will probably elicit activity at the boxoffice, but except for diehard fans it will prove a pretty tepid Halloween trick.