Our aim is entertainment, says 'The Shakespeare Plays' producer. Also, a battle won in the PBS funding struggle

Riddle: When is a tetralogy a group of seven? Answer: On public television starting this Sunday, when Shakespeare's famous four-part War of the Roses cycle begins its seven-part presentation.

Get out your pencil now - from Sunday, March 27 (12 noon-1:30 p.m., check local listings) through Sunday, May 2, the fifth season of ''The Shakespeare Plays'' will present ''Henry VI, Part One,'' ''Henry VI, Part Two,'' ''Henry VI, Part Three'' and ''Richard III.'' The Henrys will each be in two parts; Richard in one part.

Summing up - the fifth season, four plays, seven parts, sixteen hours. That's a lot of Shakespeare!

But not too much for Shaun Sutton, the new executive producer of the Shakespeare canon, for many years in charge of BBC drama. He has taken over as executive producer from Jonathan Miller (who produced ''Henry VI, Parts One and Two''), who took over from Cedric Messina, who was primarily responsible for getting the BBC/WNET 37-play Shakespeare folio on the air in the first place five years ago.

Recently I chatted with Mr. Sutton, who was here from London to stir up more interest in the ''Shakespeare

Plays'' - PBS has moved it into a noon Sunday slot rather than the prime time it formerly occupied. We talked in his suite at the Algonquin hotel, New York's staid refuge for visiting literati.

''I'll be doing the last 14 Shakespeares,'' he said, ''and above all, I want them to be entertaining. People tend to forget that Shakespeare was a working playwright in a working theater. He wrote and rewrote because he set out to entertain in the true sense of the word. And his audience was mostly semi-literate. They must have been very good listeners because sometimes 3,000 of them stood there in the rain at the Globe Theater. They didn't come for an intellectual feast; they came to be entertained.''

Mr. Sutton plans to do more or less traditional productions, but he is not against others doing more outlandish versions.

''The funny thing about Shakespeare is that you can do almost anything to him and he still survives. But I felt right from the start that if we are going to reach vast new audiences with Shakespeare for the first time, we must have some sort of policy of presentation. We needn't be stuffy but we should at least stay within the scope of Shakespeare's own knowledge and imagination. Let the costumes and and backgrounds be anything that Shakespeare himself might have imagined, as far back as we want but not beyond the 17th century.

''Over the years I've done Shakespeare every way - modern, Victorian, Elizabethan. That works in the theater. But there is something about the TV screen, a sort of conservatism, perhaps because it derives its look from the cinema, which impels audiences all over the world to expect a real look in its dramas.''

Despite this, Mr. Sutton has chosen to do all of the ''War of the Roses'' plays on one set. ''It looks a bit like a children's playground. You may be shocked when you first see it, but after a while you don't even notice the old boards and staircases nailed together with only a banner here and there to identify the location. Otherwise we would have had to spend millions on sets and locations. My feeling is you either do it as Olivier did it, spending millions, or you do it in a studio as we did it - there's no room for compromise.''

Does Shaun Sutton believe that the 37 ''Shakespeare Plays'' canon will prove to be the definitive Shakespeare? He smiles and shakes his head. ''Anybody who sets out to do a definitive Shakespeare is fooling himself. There is no such thing. Anybody who does 'Hamlet' is certain that his 'Hamlet' is the definitive 'Hamlet.'

''Certainly these productions will be seen in 40 countries and are being used on cassette in hundreds of schools. But are they the definitive Shakespeare? Will they last forever?

''I'll settle for the year 2000.''

WNET/NY makes it

The crucial money crunch that threatened WNET/NY's future programming has been relieved.

Troubled WNET claims responsibility for around 40 percent of PBS prime-time programming. And last month WNET's gutsy president, John Jay Iselin, put his career on the line when he sent a letter to 300,000 station members asking for an emergency $1 million to prevent the station from cutting back on programming plans. Personally under fire for some innovative but seemingly unsuccessful attempts to put the station into the black, he termed the request ''a vote of confidence'' in himself.

Such shows as two upcoming series on ''Civilization and the Jews'' and ''The Brain'' were endangered. WNET is also the producing station for ''Live From Lincoln Center,'' ''The MacNeil/Lehrer Report'' and ''Great Performances.''

Well, the returns are still being tabulated, but my sources within the station tell me that it now appears that Mr. Iselin's unprecedented appeal will be answered by donations totaling more than double the $1 million he requested.

The figure is already over $2 million, with more coming in every day.In addition, WNET's latest membership drive, which ended just this past Sunday, has set an all-time station fund-raising record with a total of $2.36 million pledged.

So with the injection of fresh money, it looks as if Iselin, WNET, and its top quality programming will remain an important part of the national PBS scene. Corporations back PBS

Meanwhile, big business has officially joined the battle to retain public broadcasting as a major force in American culture.

While PBS supporters in Congress and among the public fight to keep government appropriations from being slashed, some of the nation's top corporations have joined forces to support what Rep. Timothy E. Wirth (D) of Colorado has called ''a national treasure'' - PBS. The group has been meeting privately for several months and has finally decided to go public, calling itself Corporations in Support of Public Television. CSPT includes such major US companies as J. C. Penney, AT&T, Atlantic Richfield, Exxon, Mobil, Chevron, Danskin, Ford, GE, IBM, Xerox, and Gulf Oil.

Although at this writing they are still awaiting general membership approval, it appears that the first official meeting of the whole group will be held on June 23 in Washington.

Aims of the group? As announced, to encourage new corporate underwriting, provide a forum to share underwriting experience, provide information for new underwriters, serve as a PBS sounding board.

But unofficially it is corporate America's way of telling the President two things - that it does not want PBS to be stifled, and that it is responding to President Reagan's request for more private sector participation. Of 'Roots' and 'Thorn Birds'

''Inflating short stories into 6- or 10-hour series is not the magic formula, '' says Stan Margulies, producer of the fabulously successful ''Roots'' and now of ''The Thorn Birds.'' When I spoke with him he was still at work on another miniseries, ''The Mystic Warrior,'' based on American Indian life.

''Both 'Roots' and 'Thorn Birds' were stories we simply could not get down to the two-hour form,'' Mr. Margulies explained.

But more TV executives seem to be looking to the long form now. Why so many miniseries?

''What makes them popular right now is successs. 'The Blue and the Gray' did well, as did 'Shogun' and 'Marco Polo.' 'The Winds of War' was spectacular. Even 'The Executioner's Song' did well. Miniseries have become event TV.

''For a miniseries to succeed there has to be some special ingredient - either cast, best seller, or production values. But remember that the same seeds of success are also the possible seeds of catastrophe because the network has to commit immense amounts of money and many prime-time nights. If it opens big, every succeeding night will get bigger. If it opens poorly, chances are every succeeding night will get smaller. The network will be locked into a show that is going no place.''

The Thorn Birds (ABC, Sunday, 8-11 p.m., Monday, Tuesday, Wednesday,) based upon Australian Colleen McCullough's best seller about a backsliding priest, has already been denounced by church groups because of ABC's ''bad taste'' in choosing to air it during Holy Week. But it can easily be faulted on its own - it is a lush, soapy mix of temptation and salvation, bound to be offensive to many people although it also seems bound to attract millions of viewers because of its slick storytelling style. And once again the ABC public relations and promotion departments have gone to great lengths to make certain the world is aware of the miniseries.

The legend of the thorn bird is, for Mr. Margulies, a very moving legend. ''As far as I know the author invented it. The legend is that there is a thorn bird which seeks out the thorn tree and impales itself upon the sharpest thorn while singing its one final song. In that moment, it outcarols the lark and the nightingale, and the world listens. I suppose it means that success, or a sense of accomplishment, is sometimes bought at the price of great pain. That's a lesson we can all be reminded of. There ain't no free lunch.''

Might ''The Thorn Birds'' miniseries turn out to be Mr. Margulies's personal thorn bird?

''I hope not. I certainly plan to go on working. I have so many other miniseries I want to do. . . .''