CBS News dropped a major bombshell yesterday when it announced the names of the anchors for its upcoming afternoon daily news program: none other than "60 Minutes" superstars Mike Wallace, Morley Safer, Harry Reasoner and Ed Bradley.

The announcement was as unexpected as if the producers had signed Bill Paley himself, especially considering that advance rumors had such frothy show-biz types as Marlo Thomas under consideration.

A CBS News spolesman said a title for the show, "Up to the Minute," has also been chosen and said that the four men would anchor the show on a rotating basis, each of them taking one week each month of the program, set to air at 4 p.m. daily. The signing of four such luminaries means the program already ranks as one of the most substantial and potentially impressive efforts ever expended in daytime network television.

The program is clearly a way of opening up more territory for the news division and also may be paving the way for the one-hour nightly newscast that CBS News president Bill Leonard has repeatedly said is nearer to his heart than his lungs are.

Bringing in the heavy hitters is obviously also a ploy to make it all but impossible for CBS network affiliates to bypass the show in favor of their own local fare; currently, many affiliates forego the CBS 4 p.m. show -- reruns of "One Day at a Time" -- in order to air their own profitable old movies or syndicated talk shows that lead into their local newscasts.

Edwin W. Pfeiffer, general manager of CBS affiliate WDVM-TV here, had previously expressed reluctance to carry the show. Yesterday, with the news of the four horsemen as anchors, he was impressed but not thoroughly converted. "My God, that sounds as though it could be great," he exclaimed, but then he said he wondered how the hard-news elements will be combined with a previously announced regular feature, Captain Kangaroo's parenting advice.

Pfeiffer said he does not want to move the station's currently successful "John Davidson Show" from its 4 p.m. time slot or opt for the one-hour version of the show that the syndicator makes available. "The best of all worlds," Pfeiffer said, would be if the network would let him delay the airtime for the CBS News show until 11:30 the next morning. It is doubtful that will appeal to the network that christened the show "Up to the Minute" . . .

Elsewhere in the network new business, CBS News reluctantly confirmed that correspondent Richard Threlkeld, who often does the "cover story" on the acclaimed "Sunday Morning" show, is leaving the network for ABC News. Threlkeld, on assignment and not immediatley available for comment, has been with CBS News since 1966 and was considered a key talent. But an insider said Threlkeld felt he was getting lost in talent-heavy CBS News. At ABC News, they don't quite have that much of a crunch, heavy-talent wise . . .

An ABC News spokesman said late yesterday that the Threlkeld hire was true. But he declined to confirm an NBC insider's claim that ABC News has made "a great offer" to NBC correspondent Linda Ellerbee, whose contract runs out at midnight Sunday. The ABC spokesman said "there have been discussions but an offer has not been made."

And Lloyd Dobyns, whose contract runs out at the exact same moment (the two of them anchored the reverently remembered "Weekend" show when it went, briefly, to prime time), said yesterday he is "still talking" to NBC News about renewal but also talking to CBS and ABC.

Dobyns' agent happens to be his wife. According to Dobyns, the network made him an offer not hard to refuse: three more years at the same salary. Dobyns said the offer to Ellerbee was similarly stingy.Come midnight Sunday, NBC News could lose two of its brightest, sharpest, most literate reporters. Only NBC could klutz things up so royally. "I'm tired of being insulted," Dobyns grumped.

Dobyns, 45, has been with NBC News since 1969, and anchored "Weekend" from its beginning as a monthly magazine in October 1974; Ellerbee, a mere 37 years old, joined on in 1975 as a Washington correspondent. Both are highly regarded by top news executives for their ways with words, not a common talent along the old correspondent trail.