There's a lot of talk these days about a fourth television network.

Two advertising agencies, Ogilvy & Mather and Benton & Bowles, recently announced that they had entered into an arrangement with Metromedia Television to sell a certain number of hours of prime-time programs that will be sold to individual stations. The arrangement has a pretty catchy name - MetroNet.

Operation Prime Time is another entry in the fourth television network sweepstakes. A number of independent stations have commissioned a mini-series produced by MCA-TC, based on Taylor Caldwell's best seller, "Testimony of Two Men."

Norman Lear, rebuffed by the networks when he tried to sell them "Mary Hartman, Mary Hartman," flew a number of station owners and managers into Los Angeles for a sumptuous dinner at his Brentwood home in the summer of 1975, and in effect created his own little network of individual stations of "MHMH."

Paramount Pictures recently bought controlling interest in the Hughes Television Network. No one seems quite sure what it intends to do with it.

And to add to the ferment and talk about a fourth television network, a mysterious firm named Satra, which has specialized in American-Soviet trade for the past 25 years, is making noises about having obtained the North American television rights to the 1980 Summer Olympics in Moscow.

All these developments look great on paper. But at the risk of being a cynic about a desirable development - namely, more diversity in prime-time television programming - I must confess that phrases about a fourth network afflict me in much the same manner as does talk about the "revolution of cable television."

I try not to hold my breath waiting for it to happen. I harbor, despite my desire to believe otherwise, the sensation that it is all the lull before the lull.

Much of this talk is paraded before us as an important breakthrough in programming diversity. When you get down to the fine print, however, what is really at issue is creating more opportunity for advertisers to buy time on television.

That is not as easy as you might think. Time has been virtually sold out at the three television networks for some time when it comes to buying commercial minutes. As a demonstration of how network television can be an upturned cornucopia of money, NBC finished third in the ratings last year and still racked up record profits.

I don't doubt that a lot of advertising agencies and the clients they serve are anxious to find some alternative to much of the fare now presented on the three networks. But what is really at issue is not primarily an attempt to seek better programming. It is an attempt to seek more programming in which advertising can be placed.

I am not putting the knock on such efforts. I applaud them. But I urge caution on anyone who thinks that creating more programming is synonymous with creating better programming.

Talk about a fourth network is much too grandiose in terms of what we can expect in the near or the long term. Despite a lot of argument to the contrary, the people who plan programs at the three networks do not deliberately try to encourage writers and producers to deliver schlock.

It is just tough to fill the schedule of a network and a local station. In a speech he gave in Washington last summer, Eric Sevareid quoted a station manager who said that, in terms of television's enormous appetite for product, there wasn't even enough mediocrity to go around.

Instead of talk about a fourth network, we should concentrate our attention on attempts like the one recently made by the Mobil Oil Corporation to bring us its 10-part series of famous explorers, "Ten Who Dared."

The first episode, based on the voyages of Columbus, was seen on 46 stations around the country. It did quite well in the ratings on independent stations and even better on stations affiliated with one of the three commercial networks.

Of the 46 stations that are carrying "Ten Who Dared," 29 are affiliated with one of the three networks. More than half of the network affiliated stations that ran the first episode of "Ten Who Dared" were from NBC. They had a good reason, NBC that night had a turkey on called "Bunco." Of the 67 rated programs last week, "Bunco" finished 67.

It strikes me that what Mobil did is probably a better way to offer diversity to viewers than trying to create a fourth network. Pick a product of quality - this one produced by the BBC and Time-Life Films - and peddle it yourself to any station, independent or network-affiliated, that wants it. It doesn't look on paper as grand as the concept of a fourth network, but as Mobil has proved with "Ten Who Dared" there is a substantial audience out there for quality programming. You don't need a fourth network for that kind of programming. All you need is a corporation like Mobil that knows what it wants and how to get it on the air.