By the time ABC TV wraps up its coverage of the 104th Kentucky Derby at 6 p.m. tonight, the network will have devoted 4 1/2 hours of air time to an event it has been promoting furiously for weeks as the most exciting two minutes in sports."

At a similar ratio a Super Bowl would command about 405 hours, or a little more than 16 days, of attention. A seven-game World Series would merit 24 hours a day for 112 days - with time off, presumably, for good behavior and commercials.

Such comparisons are frivolous of course, but the amount of time ABC has devoted to building up its onehour telecast of Derby Day itself (WJLA-TV-7, 5 p.m.) indicates how important it is on the networks list of sports priorities.

"Of the annual events we do, the Derby is right up there with the World Series (on alternate years with NBC), the Indianapolis 500 and the U.S. Open golf, in no particular order," said publicist Irv Brodsky. ABC - which is televising the Derby for the fourth consecutive year, and this week announced an extension of its contract through 1982 - has had a staff of 50 on the job at Churchill Downs and expects 33 million viewers to tune in at some point during today's telecast.

The social trappings, color and offbeat stories, and other items of interest only peripherally related to horse racing have been covered on "Good Morning, American," which yesterday originated from the scene of a sporting event for the first time, and an hour special called "Derby Week: An American Tradition." It was cohosted by Frank Clifford and Chery Tiegs, ABC's beauteous new "entertainment personality," last night.

Jim McKay and Howard Cosell, the embodiment of humility and hubris, take over today, with five-time Derby winning jockey Eddie Arcaro as the expert analyst and Dave Johnson the voice of Santa Anita, doing the call of the race itself.

The four leading contenders - Alydar, Affirmed, Sensitive Prince and Belive It - have been the focal point of preview on ABCs "Wide World of Sports," and will receive the thorough bred's share of the attention in 20 minutes of analysis leading up to this afternoon's 5:33 Post time.

"This is an unusual year. We could easily fill an hours before the race and make it pretty interesting because there are so many important and fascinating stories," producer Chuck Howard said yesterday, fretting that 20 minutes is not nearly enough time to do justice to what shapes up as one of the greatest Derbys.

Central topies will be the rivalry between Alyder and Affirmed; the sentimental return to preminence of Calument Farm and its octogenarian owners, Adm and Mrs. Gene Markey, who have had a record nine Derby winners but have not sent a horse to the gate in seven years, until Alydar; and the first Derby ride of 18-year-old Steve Cauthen, who will be up on Affirmed for his return to his old Kentucky home.

Unlike a golf tournament or an auto race, where the emphasis is on flitting around to the best live action, the Derby coverage is packaged almost like a scripted show. Pretaped segments dominate almost half of it.

There is room for flexibility, depending on the weather and breaking news - "the idea is to have a plan from which to depart," said Howard - but these are the segments scheduled for prerace airing:

A comparison of Alydar and Affimed, which will include their lineages, past performances, trainers and jockeys - a kind of equine "tale of the tape" - and will include footage of them as 2-year-olds (when they met six times in a thrilling rivalry) and as 3-year-olds.

Impressions of how the race will develop strategically, from trainers Laz Barrera (Affirmed), John Veitch (Alydar), Allen Jerkens (Sensitive Price) and Woody Stephens (Belive It).

A super-slow-motion segment on Cauthen's riding style, shot at Hollywood Park.

Super-slow motion footage of Cauthen-aboard Affirmed, filmed earlier this year at Hollywood Park. Arcaro will analyze Cauthen's riding style and assess Affirmed as having not only great heart but as much "presence" as any throughbred he has seen,

A McKay interview with Mrs. Markey, 85, at Calumet Farm.

Copell interview with Cauthen about the feelings of the lad from nearby Walton, Ky., on riding in the Derby for the first time.

Howard and director, Chet Forte, will have 15 cameras at their disposal.They will be deployed as follow: two directly behind the starting gate; one at the end of the clubhouse roof to pick up turns three and four; two at the finish line; one law in the first turn; one in the backstretch; two in the winner's circle, where McKay and Cosell will open the show and conduct postrace interview; two in the paddock; one on the roof of a truck overlooking the rose garden; one on top of the tote board, to capture the infield scene and post parade; one in a helicopter overhead, and one at Calurnet Farm, where correspendent Andrea Kirby will interview the Markeys, win or lose.

"The trick," says Howard, "is to find the common denominator between the real horse-racing aficionado, who has watched all the Derby hopefuls develop for more than a year, and the person who never goes to the track but watches the Derby and perhaps the other Triple Crown events on TV.

"From the time the horses are sadled and leave the paddock, it flows.Everybody wants to see the post parade and the singing of 'My Old Kentucky Home.' We follow the horses to the starting gate, break for a commercial and when we get back they're just about ready to start the race."