In the four years since the introduction of home videocassette recorders (VCRs), just under a million Americans have been lured into buying video hardware, the actual VCR units. But a boom is in the air: Industry experts predict VCR sales of over 600,000 this year and 12 million by 1985. The main reason is the increasing (and cheaper) availability of software, or programming, including big-name films like "Patton," "M*A*S*H" and even "Deep Throat."

VCR owners can buy or rent hundreds of movies, sports events and old television shows from specialty catalogues around the country. At least two national videocassette "clubs" operate on much the same basis as mail-order record clubs. And beginning in the fall, videocassettes of such blockbusters as "Saturday Night Fever" and "The Godfather" will be as close as the nearest Fotomat island.

Even in the Washington area, where VCRs have not caught on as quickly as might have been expected in a traditional luxury-item market, the word is spreading. According to one estimate, there will be 200,000 VCRs in the metro area by the end of this year.

It is, as the vice-president of Washington's newest videocassette library points out, a question of entertainment and quality.

"Our philosophy is that if someone buys a VCR machine that costs him somewhere between $900 and $1,100, he ought to have a choice of films that he wants to see, not what television wants him to see," says Dick Dacy, marketing vice president of the Hyattsville-based Video King company.

"And we're out to beat the pirates. If we make films available cheap enough, it will combat the pirates. Who would pay a higher price for an illegitimate tape when you can get the uncut version and better quality?"

Video King, which will be in full operation by the end of the months, is a rental library with some 200 titles: recent features (some never shown on TV or cable), classics, musicals, rock concerts, war films, children's features, horror movies and X-rated films.

For an annual membership fee of $100 and service charges of perhaps $200 a year, Video King members receive 48 videocassettes a year in pairs, each for up to 10 days. "It's two double-features a month for 12 months" is how operations vice president Bob Sorkin puts it. "We offer everything but the popcorn."

Video King plans to "go national," with a toll-free phone number and Visa, Master Charge and American Express. By the mid-1980s, when VCRs really take hold, Sorkin hopes Video King will dominate the mid-Atlantic and Eastern region.

(Video King, like all major videocassette companies, handles both BHS and Beta tapes. Sony, one way or another, manufactures VCRs for Zenith, Sears, Sanyo and Toshiba as well as its own Sony Betamax; all these models use Beta tapes. The Japan Victor Corporation, known as JVC, manufactures Panasonic, Quasar, Magnavox, Sylvania, RCA, Philco, Histachi and GE; these use VHS cassettes.)

The Fotomat videocassette campaign is already underway in southern California. Besides the Paramount Pictures contract that gives them both "Godfather" films, "Saturday Night Fever," "Looking for Mr. Goodbar," "Marathon Man" and others, Fotomat will stock older classics like "Citizen Kane" (apparently a nation-wide staple), "Sunset Boulevard" and "Shane." Fotomat customers will be able to rent any videocassette for five days for $11.95. One-cassette films will be sold for $49.95 and two-cassette films for $59.95.

According to Fotomat program manager Steve Wilson, another major catalogue will have been acquired by the time the videocassettes are available in the Washington area.

The Maryland Center for Public Broadcasting in Owings Mill both rents and sells videocassette copies of PBS-originated shows like "Wall Street Week" and "Consumer Survival Kit." A half-hour show can be bought for $100, or rented for $25 a week.

The biggest demand so far, says Fran Johansen, has been for the "Americana/music" programs like "Jazz Fest" and "Grass Roots." Another favorite is the "Basically Baseball" series, four half-hour instructional programs produced with the aid of the Baltimore Orioles at their Florida training camp.

The Audio Center in Bethesda has about 200 titles for sale, none for rent. One-cassette features cost $59.95; two-cassette features $79.97. Their biggest sellers, according to a salesman, are "Hello, Dolly," "Patton" and "The Story of O." Of the two national clubs, one - Video Club of America - only sells videocassettes, and the other - VidAmerica - only rents.

Video Club of America, located in Farmington Hills, Mich., requires a one-time $10 membership fee, and offers an initial "buy-one-get-one-free" fringe benefit. They have 133 titles in stock, and will add 75 more to their catalogue in September. One-tape features cost members $54.95; two-tape features cost $74.95. Cartoons are $35 and sports events $30.

According to marketing director Rob Kantner, Video Club last year sold about 2 million video cassettes, and so far this year is selling at twice that rate.

Video Club of America is the direct-mail marketing arm of Magnetic Video, the grandfather of the business. Magnetic Video was founded as an audiotape services-and-products firm in 1969, and began videotape duplication for industrial clients in 1972. In 1977, in a daring and controversial move, Magnetic president Andre Blay contracted with 20th Century Fox for the duplication and distribution of 50 major motion pictures. By last November, business was so good that Fox bought the firm for $7.2 million, in cash.

VidAmerica of New York also requires a one-time $10 fee; rentals range between $9- $14 for a week's lease. A contract with United Artists has brought VidAmerica such titles as "Last Tango in Paris," "Semi-Tough" and "Rollerball"; they also have classics like "Casablanca."

"We're patterned exactly like a record club," says general manager Dick Kelly. "You get a new guide every two months, each with 20-25 programs in it. But unlike those clubs, we are non-obligatory - there is no obligation to use your membership."

VidAmerica also has a purchase option; the original rental fee is applied toward the purchase price (about $48.50). In general, only the older films are for sale; the newer titles are under copyright restrictions.

VidAmerica is getting the benefit of a promotional campaign by Sony, which bought 50,000 packs of premiums and memberships from VidAmerica and is giving them away with newly-purchased Sony Betamaxes.

Specialty outfits across the country range from Brentwood Video (fewer than 10 gay X-rated films) to Champions on Film (35 instructional tapes on tennis, swimming, etc.) to the Emory Medical TV Network (400 titles).

Astrovideo of San Francisco has dozens of half-hour ($29.95) and hour-long ($39.95) rock concerts by Ike and Tina Turner, Delaney and Bonnie, Bo Diddley, Helen Reddy, etc. The Nostalgia Merchant in Hollywood has over 600 titles in its active catalogue, and currently sells "a minimum of 5,000 tapes a month," according to president Nick Draklich. The June issue of Videography magazine lists 113 software distributors of all types.

For the time being, however, the software market is indeed soft. Industry figures show that, even at prices between $17- $25, three times as many blank cassettes are sold as pre-recorded.

Apparently VCR owners still prefer to copy "Charlie's Angels" rather than buy Charlie Chaplin. CAPTION: Illustration, no caption, by Juanita Mondello for The Washington Post; Picture, Al Pacino in "The Godfather."