First came "The Exorcist," at $124 million worth of worldwide rentals one of the most successful films ever made. And as big as that was, that's how big a disaster its sequel, "Exorcist II: The Heretic" is turning into.

So now, like a hard-riding troop of cavalry, "Exorcist III" and even "Exorcist IV" have appeared on the horizon, a pair of stouthearted attempts by Warner Brothers to salvage a fiasco that is threatening to become as big as all outdoors.

"Exorcist IV" has over 100 separate and rapidly-made changes from "The Heretic," featuring a prologue of star Richard Burton discussing exorcism over a series of stills recalling the original film. These changes have been made in a great hurry to prepare for the film's July 12 Japanese opening, its first overseas.

It's not like "Exorcist II" is the worst film imaginable but rather that, as the accurately ascerbic John Simon put it with its $11 million budget and large-scale studio publicity and release. "There is a very strong probability that 'Exorcist II' is the stupidest major movie ever made."

"I've been in the business now for 10 years and never have I expected so much in anticipation of a film and gotten so little," says the K-B chain's Ron Goldman, who is running the film in three of his local theaters. "I wouldn't let John Boorman direct one of the little exploitation films I make," decidedly non-chic items like "Brotherhood of Death" and "Black Gestapo."

Out in Hollywood, where he has been awfully busy repairing the damage, a pragmatic-sounding Boorman, whose previous work included "Deliverance" and "Point Blank," is well aware of what he hath wrought.

"I made a lot of mistakes, that's quite evident from monitoring various screenings," he says with a pleasant British accent. "The audience was terribly upset, the film aroused terrible hostility, the theater managers were afraid to wear their tuxedos for the opening, they were afraid they'd get lynched."

Goldman and other exhibitors are upset because the mechanics of the film industry meant that they had to put up large sums of money - in excess of $150,000 in his case - in order to get a film which none of them had even had a peek at beforehand.

Even more painful is the fact that the film's strong showing in its opening days, before word of mouth got around, indicates that if "Exorcist II" had even halfway delivered it would have done very, very well at the box office.

Trying to capitalize on this initial splurge, the studio is running ads for the film saying that "In only five days 'Exorcist II' has broken mor attendance records than any other motion picture in Warner Brothers history!"

So far, so good, but the dropoff from those heights has been awesome. In its first three days in 81 theaters in the New York area. "Exorcist II" did $669,027 worth of business; by the first three days of the second week the figure had dropped to $313,850. In Washington the drop for six theaters was similar, roughly from $81,000 to $31,000, with one theater, the Fine Arts, plunging from $16,146 to $3,704.

Boorman was at home in Ireland when the bad news hit, but was immediately deluged with phone calls advising him to do something "to prevent a new Civil War breaking out in America."

"I would've like to refer the whole thing to the United Nations, tell them to put in a task force," he says wryly, but instead he phoned in approximately three minutes of cuts from the more risible bits at the end "to avert riots," a version he now calls "Exorcist III" and which has since been rushed to the 700-plus theaters playing the film.

"I was terribly crushed, I'd worked on the film for two years, put a great amount of care into it, and the response was mortifying," Boorman says of the initial response. "I considered suicide for about 3 1/2 minutes, then I decided to come back and try to correct my mistakes, to remake the picture. Not just recut it, remake it."

So the director flew to Hollywood and began work on the now-completed version he calls "Exorcist IV," in which he has eliminated 20 minutes from "Exorcist II" and added 10 new minutes. Unfortunately, Burton's character doesn't survive the new cut. "We sacrified him," Boorman says dryly, "for the sake of the picture."

Boorman is able to joke about all this because he feels bolstered by a sneak screening last Sunday in Westwood - that "Exorcist IV" is at long last a success.

"It was chalk and cheese," he says, "in great contrast to the booing, hissing and throwing things at the screen that went on before."

"I'm learning from the audience, letting 3 million people telling me if I'm wrong guide me," he explains. "I feel that I've at least conducted an exorcism of the film, gotten rid of the demons that crept in."

"Exorcist IV" will at least be the version that goes into foreign release - where "Exorcist I" made a third of its money - but the last word from a Warner Brothers spokesman was that besides the lopped-off ending "no further changes will be made in the film that's being shown now in the United States."

As for Boorman's own final analysis of the trouble with "Exorcist II: The Heretic," he lays the problems mainly to his own misjudgment:

"I felt I could take a captive audience and lead them into a visionary and spiritual adventure story. They really didn't want that, they wanted just the gore. We created the arena, we showed them the lions, we just didn't throw them enough Christians."