Eight years have passed since Fox tossed its hat and a significant chunk of chairman Rupert Murdoch's money into the ring and launched a fourth broadcast television network - to the din of snickering from skeptics.

The disbelievers have been silenced as Fox has grown into a respected player that has managed to steal pro football, the Emmy Awards and even some affiliates from NBC, ABC and CBS.Hoping to capitalize on and repeat Fox's success, two more broadcasting giants are preparing to enter the fray. Warner Bros. will launch its WB Network on Jan. 11, while the United Paramount Network, the programming arm of Paramount, will begin Jan. 16.

Both WB and Paramount are following Fox's lead in starting slowly. WB will air its four shows on Wednesday nights; Paramount has tapped Monday and Tuesday nights to show five shows, headed by "Star Trek: Voyager."

The programs will be shown over independent stations nationwide. In Utah, the WB shows will run on KOOG-Ch. 30 and Paramount's on KJZZ-Ch. 14.

While it may seem far-fetched that a fifth - or even a sixth network - can carve out a viewer niche, Warner Bros. and Paramount officials maintain that they will offer quality programming that will get them noticed.

"We have a uniqueness to our programs that viewers will look at and say, `Wow, that's what really made Fox cool in the beginning!"' said WB Network president Jamie Kellner, who was a Fox executive when that network began in 1986.

The deregulation of the broadcast industry - allowing networks to not only broadcast shows but make and own them as well - means that better shows will be offered, officials say.

"What we are doing is programming with quality production values, equal to what the networks are doing," said Leonard J. Grossi, senior executive vice president for United Paramount.

The WB shows are designed to appeal to younger viewers - the same audience Fox eventually targeted to establish itself. It is looking to attract 12- to 34-year-old viewers with its four offbeat-charter comedies.

United Paramount Network (UPN) is zeroing in on 18- to 49-year-old men. Each of its five shows is decidedly guy-oriented.

Kellner said that WB plans to start airing shows on Thursdays beginning in August and children's Saturday/weekday morning programming in September. Grossi said Paramount plans to add a third night by mid-1996. And both services hope to be seven nights by the turn of the century.

Both networks will be seen initially in about 80 percent of the country, though 25 percent of the WB coverage can be attributed to its availability on the Chicago-based superstation WGN. That 80 percent contrasts with the 96 percent to 98 percent coverage enjoyed by ABC, NBC, CBS and Fox.

Yet, as Bill Croasdale, president of the national broadcast division for the ad agency Western International Media, points out, "Eighty percent is an acceptable launch-time number."

It remains to be seen, of course, just what percentage of those 80 percent will be regular (or even occasional) WB and UPN watchers.

To use Fox once again as the yardstick, that network started in October 1986 with the ill-fated "Late Show Starring Joan Rivers."

Once Fox started running prime-time programs in 1987, the shows were a demographic mishmash and included "Mr. President" with George C. Scott, "Women in Prison" and "Werewolf." Fox's evolution to a youth-oriented program network took a few years.

By contrast, Warner Bros. and Paramount had carefully measured targets in mind from the outset, with shows tending to be more offbeat than what was available on the Big Four.

Croasdale has watched most of the shows premiering on the two new networks and is surprisingly bullish on the chances of survival - particularly UPN, because of its tie to "Star Trek."

"I honestly think both of them will succeed, albeit on a much smaller scale than the other networks," Croasdale said. "You can compare them to Fox, but Fox has a big advantage in having been there first and establishing a strong station lineup. The other guys are forced to scramble for what's left over."

Somewhat more pessimistic is David Davis, an analyst with the media buying firm of Paul Kagan Associates. He believes that one network or the other will survive and perhaps even thrive - but not both.

"It's mathematically possible for both to make it, but a more likely scenario is a merger or a strategic alliance kind of like when the two comedy networks joined forces and became Comedy Central," Davis said.

The fact that Paramount has a "franchise" program in "Star Trek: Voyager" makes them a favorite, Davis said. "But Warner Bros. has excellent programming skills. I don't see either one of these guys failing miserably, although both are going to eat a lot of money."

*****

ADDITIONAL INFORMATION

THE WB NETWORK

WEDNESDAYS

The Wayans Bros. (7 p.m.): Shawn and Marlon Wayans star as two "very different" brothers living in New York who have a taste for get-rich-quick schemes. Premieres Jan. 11.

The Parent 'Hood (7:30 p.m.): Robert Townsend plays a university professor who struggles to live in harmony with his opinionated wife and four daffy kids. Premieres Jan. 18.

Unhappily Ever After (8 p.m.): A divorced father (Geoff Pierson) is forced to cope with sudden poverty after his ex-wife (comic Stephanie Hodge) takes him to the cleaners. From "Married ... With Children" co-creator Ron Leavitt. Premieres Jan. 11.

Muscle (8:30 p.m.): A serial comedy set inside a trendy New York fitness club that's styled after the series "Soap," featuring an eclectic group of clubgoers and employees. Premieres Jan. 11.

UNITED PARAMOUNT

MONDAYS

Star Trek: Voyager (8 p.m.) The fourth incarnation of the "Star Trek" television universe follows the adventures of the U.S.S. Voyager as it journeys from the far reaches of the galaxy back to Federation space. Premieres with a two-hour special on Jan. 16.

Platypus Man (9 p.m.): The producers of "The Golden Girls" built this series around stand-up comic Richard Jeni - playing a guy who hosts his own cable cooking show. Premieres Jan. 23.

Pig Sty (9:30): A comedy featuring five sitcom unknowns who share a two-bedroom apartment in New York City and behave as if they're auditioning for a part in the next "Animal House." Premieres Jan. 23.

TUESDAYS

Marker (8 p.m.): Producer Stephen J. Cannell is behind this series that's filmed in Hawaii and stars Richard Grieco ("21 Jump Street") as a guy whose estranged father has died and left him a mysterious series of debts to repay. Co-stars Gates McFadden of "Ster Trek: The Next Generation" fame. Premieres Jan. 17.

The Watcher (9 p.m.): An anthology series set in Las Vegas starring rapper Sir Mix-A-Lot as an enigmatic man through whose eyes we "witness the microcosm of America." Premieres Jan. 17.