You can't blame a Denver fan if he looks out a window these crisp late fall mornings and thinks he is in sports heaven.

Just four years ago, this city had a struggling American Basketball Association franchise, a mediocre National Football League club, a representative minor-league baseball team and an obsecure minor-league hockey squad. The outdoor stadium was unattractive and the indoor arena had a capacity of just 7,000.

Now, the local sports fanatic can watch the high-flying Denver Broncos dominate the NFL in a beautiful 75,000-seat stadium or he can keep his fingers crossed as the Denver Nuggets challenge for the NBA title in a 17,000-seat arena or he can pray that the Colorado Rockies of the NHL will improve.

Or he can buy season tickets for this city's latest pro top, the former Oakland A's of baseball's American League.

Of course, there are a few legal problems to be taken care before the A's become Denver's own, but that hasn't dampened the enthusiasm of this fastgrowing metropolis. Instead, citizens already have added another notch on their sports franchise scorecard.

Only eight other cities can now boast of Denver's sports grand slam of hosting teams in the NHL, NBA, NFL and major league baseball. That's not bad for an area that once wondered if pro teams would ever recognize the beauty of a Rocky Mountain high.

Until two weeks ago, the Denver sports fan hadn't even thought about a baseball team. If the A's moved, he had read, it would be to New Orleans or maybe even Washington, D.C.

But that was before a former Notre Dame basketball coach told one of this city's richest men how he could acquire the A's for a few million dollars, nothing really when your wealth has been set at a minimum of a billion dollars.

Johnny Dee, the former Irish coach who now is a city auditor here, knew both Marvin Davis and A's owner Charlie Finley. And he knew that Davis had thought for years about owning a sports franchise, although he had kept his dreams in the background.

So Dee arranged for a meeting between these two self-made men and Finley soon had himself a buyer at a mere $12.5 million.

"When Marvin Davis makes up his mind, he moves fast," said Dick Eicher of the Denver Chamber of Commerce. "I mean, he's almost a legend in his times around here."

Davis heads his own oil company, reportedly the largest independent oil organization in the country. He drilled 311 wells last year, behind only Shell and Amoco. He will drill at least 400 wells this year, which could make him No. 1.

He also runs a bank, owns a radio station and thinks nothing of calling oh say, Anwar Sadat for a chat about international oil problems. And when Jerry Ford, formerly of Michigan and the White House, comes to town, he stays at the Marvin Davis mansion.

"If you could order a perfect owner from a Sears Roebuck catalog, Marvin time Denver sports writer Bob Collins. Davis would be that man," said long "As they say, he has slightly more money than he'll ever need and he isn't on a big ego ride."

Indeed, Davis is no threat to Ted Turner or George Stinbrenner for headline hogging. He says he will hire competent baseball men to run his new toy and then return to the sidelines, where he prefers to stay.

Why did the large (6-foot-4", 300-pound) self-described former average athlete take on the headaches of a major-league team?

"In life, you get stereotyped in business, you do everything motivated for business," he said before heading for a Palm Springs weekend. "Sometimes you want to do something for fun when you're 52 years old.

"I've been a baseball fan all my life. I've followed it and played some, although not too well. But I've enjoyed it."

Denver is the kind of city where a man can be as rich as Davis and still be one of the guys. He even has his home number - and those of his children and household help - listed in the local phone book.

This is also a good baseball city, as shown by its loyalty to its minor-league teams. In 1949 the Denver Bears, who have been affiliated with such major-league clubs as the Yankees and the former Washington Senators, drew almost 500,000 fans, which is more than Finley's team lured last season. The current Bears attracted just under 300,000 people last year while winning the American Association title.

Bob Howsam, who has built a powerhouse with the Cincinnati Reds, was responsible for much of the club's success during the 1950s. He also constructed a smaller version of the current city stadium and he started the Bronco football club before running into financial problems and selling out.

Now Davis will be out to prove that Denver can go bonkers over baseball just like it has football. Already his office has received requests for 200 season tickets and radio and TV stations are lining up, trying to obtain rights to televise and broadcast games.

There are some headaches ahead. Finley has to overcome his problems with Oakland officials and Davis is already far behind in putting together an organization. And springs in Denver are cold and rainy, hardly conducive to enjoyable baseball watching.

"I'm prepared to spend $5 million on our first year's budget," Davis said. "That is in addition to the purchase price. We will be determined to put together a competitive baseball team."

Around here they say when Marvin Davis makes up his mind, he rarely fails.