NEW YORK (CNN/Money) - Fox is banking on a few starlets to sing their hearts out this week -- and give the floundering network a much-needed ratings boost.

The fourth season of "American Idol" kicks off with a two-hour episode Tuesday and once more the News Corp. (Research) unit finds itself reliant on a small cadre of crooners to draw the audiences that have eluded the network midway through another television season.

Fox is mired in fourth place among primetime viewers, behind CBS, ABC, and NBC, after a string of flops.

"They have a lot riding on 'American Idol' and, hopefully for Fox, the show has long coattails," said Brad Adgate, the senior vice president of corporate research at Horizon Media, a New York branding firm.

Looking to keep the show fresh, network executives have tweaked the format. That includes adding a third night for eliminating contestants. It's a move that could pay off if ratings are strong but, as ABC discovered a few years ago with 'Who Wants to be a Millionaire,' could backfire by saturating viewers.

In years past the show has lifted Fox out of a similar ratings slump, propelling the network from No. 4 among the top primetime broadcast networks to finish the season in second place among the 18 to 49 year-old viewers that advertisers covet most, according to Adgate.

Fox executives "have gotten themselves into a mid-season dependency on 'American Idol' to pull them out of ratings doldrums," agreed Shari Anne Brill, the programming director for the ad-buying firm Carat USA.

"American Idol" has not only saved the day for Fox before, the show has been a financial boon too. The price of a 30-second commercial bought in advance tops out at nearly $660,000, making "American Idol" the most expensive network show, according to an Advertising Age survey of 2004-2005 television ad rates.

Heading into the fourth season, however, there are warning signs that the "American Idol" ratings lift that Fox once again desperately needs is far from guaranteed.

Audiences have shown this year that their appetite for reality television is satiable. Heavily-hyped newcomers like "The Next Great Champ," "Rebel Billionaire" and "Who's Your Daddy" -- all from Fox -- bombed with viewers.

Other networks have seen their reality TV debuts flameout: last week CBS, a Viacom (Research) property, cancelled "The Will" after its Jan. 8 premiere drew too few viewers.

In another ominous sign, ratings for past reality hits such as "The Apprentice," "The Bachelor" and "Survivor" dropped in the fall from prior seasons.

A glut of reality shows is not all that "American Idol" is up against. While the talent competition has made past finalists like Kelly Clarkson and Clay Aiken into mini-sensations, 2004 winner Fantasia Barrino has not been as popular with music buffs.

Last season's finale drew 25 percent fewer viewers than the 2003 finish.

Barrino's debut album, "Free Yourself," sold 686,000 copies in the first six weeks after its Nov. 23 release, according to Nielsen SoundScan figures. That's roughly half the 1.3 million that Clay Aiken's "Measure of a Man" sold and 15 percent below early sales of Kelly Clarkson's "Thankful."

"I haven't heard a Fantasia song on the radio, not that I go looking for it," said Adgate. He acknowledged the slower sales could be a warning sign. Or, he said, it could be a fluke.

This time around, however, Fox is shaking up the format, although the judges -- Paula Abdul, Randy Jackson and the caustic Simon Cowell -- and host Ryan Seacrest return.

But instead of 32 semifinalists, 24 contestants split evenly between men and women will advance out of the initial auditions. The men will compete one night, the women the other.

During the elimination episodes, "American Idol" will go to three nights a week.

If anything, the increased frequency is where Fox could stumble, said Brill, of Carat USA.

"Too much filler (programming) could alienate audiences," she said. She noted that last season Fox used special broadcasts of "American Idol" to fill holes in its lineup and also extended its elimination episode from 30 minutes to an hour.

"Too many (specials and hour-long elimination episodes) could tarnish" ratings, she said.

Still, Brill and other industry watchers say there's reason to think Fox can pull off another ratings win with this next "American Idol." They credit the network for whetting viewer appetites by letting eight months lapse between the end of the last season and Tuesday's debut.

"Fox has been very careful to protect to franchise by airing it once a year," said Adgate. "That may work again and, unlike other reality shows, that may be the one thing going for it."