A Shakey's Pizza Parlor in Muncie, Ind., becomes an unlikely focal point for the viewing nation tonight when the fourth and most moving episode of "Middletown" airs, at 9 on Channel 26 and other PBS stations. "Family Business" is more about family than business, and it is also exceedingly gratifying--an authentic shining heartwarmer, a goose-pimpler, a restorer of faith.

"Family Business," produced and directed by Tom Cohen, is a leap forward for the film documentary, if only because all one has to care about is people to appreciate its story of Howie Snider, his wife, Judy, and their eight great kids, who together try to keep a Shakey's franchise in Muncie alive and thriving. It isn't easy. Early in the film, Howie's on the phone to Shakey's corporate headquarters pleading for time to pay back $10,000 he still owes the company, while his wife surrenders to sobs nearby.

Snider, who will be 50 this month, served 20 years in the Marine Corps, including combat duty in Vietnam, before leaving the service in 1972 to be with his family in Muncie. The film was made from December 1980 through February 1981, when Snider had been in business nearly six years and was still hanging on for dear life. And it is pretty dear. Something about the Sniders reminds you of that.

The Sniders set stirring and timely new examples of family solidarity, sacrifice, love, loyalty and other values that some people fear are dying. Yet the filmmakers never seem to be pushing for effect or inflicting on the story connotations it doesn't naturally have; the Sniders aren't just statistics brought to life--they're not the Nobodies of South Succotash. They're the people that cynics in government and journalism and entertainment and big business forget are out there. They're the real America that really does exist.

Howie Snider, affably burly, bearded, gregarious, not only runs the pizza parlor and heads the family, but also, with his trusty banjo, joins 81-year-old Marjie Englehart at the piano to entertain the customers with "Sweet Leilani" and other long-gone tunes. Throughout the film, he endeavors to hold the family and the business together, and both struggles converge at an official family conference near the film's end.

Here he announces that belts will have to be tightened further, that times are going to get rough. But the session turns into more than a reading of the ledger. A son, Lloyd, breaks down when he talks about the way dad has to hustle now, compared to his walking-tall days in the corps. "I don't see it anymore. I don't see the pride you had as a Marine," he says with tears in his eyes.

But the film is enthralling not only for its major emotional moments, but also for all the beautifully observed details of this family's life, which has universalities that link it to all family life. A hundred dramatists in their thinking caps couldn't come up with a better story, and for all the time he spends at the brink, Howie Snider comes across as a sensationally indefatigable American hero.

From Muncie, where his pizza business still survives, Snider says he'll be watching the film with everybody else when it's shown on television. He's girding himself for the changes this burst of celebrity is bound to make in his life, and he doesn't sound like the type who'll go nutty over it.

"After all this dies down, I still have to live here in Muncie with the people I've known all my life," he says. "Fame is very fleeting, and I hope infamy is, too, in case some people are affected by this film the wrong way." Snider's only misgiving about letting the filmmakers into his life was prompted by a friend who reminded him what happened to the Loud family of Pasadena, Calif., who were the subject of another documentary, "An American Family," a decade ago. The Louds came apart in front of the whole country; but this is a different film and very much a different family. Says Snider, "I figured the worst that could happen was that I'd be embarrassed, and hell, I'm embarrassed every day."

Snider acknowledges that the attention has changed the family's life recently, but he says he doesn't think this will last very long or prove disruptive to his life or his family. "I had just started getting bigheaded when Time magazine said I was a lousy banjo player," Snider laughs. "Well I don't play the banjo very well, and that sort of brought me back down to earth."

Peter Davis, who was the executive producer of "Middletown," and Cohen, who produced and directed "Family Business," had Snider's complete trust, he says now. Snider's son Lyn, 21, who also appears in the film, says of the filmmakers, "They were great. After a while, it was more than a business thing. We invited them over for Christmas dinner, turkey and ham and stuff. They're a great bunch of people."

A huge controversy has, of course, engulfed the "Middletown" series' last program, "Seventeen," which has been withdrawn by producer Davis because of his refusal to go along with cuts proposed by PBS. The Sniders' overwhelming endorsement of the filmmakers casts doubts on claims by students in the "Seventeen" film that they were somehow tricked into behaving like goons, which is how most of them come off in the film (although a different filmmaking team was responsible for "Seventeen").

Some of the criticism of the "Middletown" series has questioned its validity as a sociological study--a validity never claimed by the filmmakers and an irrelevancy in the first place. What some critics are saying is, in essence, that "Middletown" isn't dull enough.

When PBS President Lawrence K. Grossman made his first ill-considered public remarks about the "Seventeen" episode, he told a reporter the program did not live up to the "journalistic standards" of PBS. To which more than one realistic PBS viewer might have responded, "What journalism? What standards?" The "Middletown" series proves that something important and valuable can also be made extremely watchable.