Four years have passed since Roberta Flack's last concert in Washington, where she launched her career, so she wanted everything to be perfect for last night's free concert with the National Symphony on the Capitol lawn.

But she had a rough afternoon: Parts of the rehearsal displeased her, the reminder of three show-business friends' deaths brought tears to her eyes and exhaustion from the last month of touring made her jumpy.

"I was so excited about working with the symphony. And also to come back and give a free concert, well that was too exciting to pass up," said Flack happily, distributing kisses and hugs to friends who stopped by the rehearsal.

The rehearsal was held early yesterday afternoon at Lisner Auditorium - with the symphony members in shorts, slacks and sports shirts, Flack's combo in jeans, T-shirts and straw hats, and the musician herself in brown and peach sun dress and dreadlocks. It seemed to be goind well.

Harry Ellis Dickson, the assistant conductor of the Boston Pops, who replaced the late Arthur Fiedler, originally scheduled to conduct last night's concert, conferred with Ray Jones, Flack's musical conductor, on the moves of the strings and trumpets. At one point Flack, who was playing the piano but not singing, got up and sang a transition in "Killing Me Softly," her smash international hit of a few years ago. The orchestra said "ah" but Flack saw something she didn't like.

"I looked up and saw frowns on people's faces. Lots of heavy blowing, a physical response that told me they didn't like something. Now my music is not loud, not raunchy or decadent," said Flack, back in her two-room suite at the Watergate Hotel.

"And that attitude is snobbish, very pedestrian. When you doing a song like "Killing Me," a worldwide classic that some people had to buy bootleg..." said Flack, at this point truly piqued.

Her anoyance was underscored by the financial sacrifice she made for the concert; according to both Flack and the symphony personnel, she accepted less than half her regular concert fee and contributed about $15,000 to the expenses of her entourage and such extras as a varied style of lighting.

"I don't think it was racist - sexist some - but it was the snobbishness of classically trained musicians. Two of the string players told me they did the same thing to Arthur Fiedler."

She leaned into a corner of the sofa, tucking her shapely legs under her and smoothing out the antique lace on her peach cotton jacket. Slimmer than in former years, she looked drained without makeup but managed to smile at some good memories of the last 11 years.

It's been that long since Flack, 39, quit her teaching job in Washington and signed on as the piano player and vocalist at Mr. Henry's on Capitol Hill, she says, for little money. A year later she was headed to stardom, and five gold albums and four Grammys in six years.

Though she was born in Black Mountain, N.C., Flack considers the Washington area, where she moved at age 4, her home.

Like many other black entertainers, she received her firt nurturing in the church, but her father also gave her classical piano lessons. At age 15, after studying in the Washington public schools, she received a music scholarship to Howard University. For six years she taught school.

Her career grew rapidly after Washington adopted her as its leading songstress in the late '60s. In 1969, a song from her debut album became a hit and later the song, "The First Time Ever I Saw Your Face," was used for a Clint Eastwood movie. It was all success for several years. Then, in 1975, she halted many of her recording and concert appearances. She was troubled by tonsillitis and reportedly an IRS bill. But last fall her long-awaited album "Blue Lights in the Basement" zoomed to the top of the pop charts.

Yet, this year, just when she was bouncing back from a three-year hiatus, her life was marred by personal loss. In January, her longtime friend and collaborator, Donny Hathaway, 33, took a fatal plunge from a New York hotel window. Six months later, Van McCoy, 39, another musical partner, died. And last month, Minnie Ripperton, 31, who once did a tour with Flack and Quincy Jones, died from cancer.

"It just wore me out. Donny, Van and Minnie," said Flack. She closed her mink-brown eyes quickly. "I'm sorry, it just makes me cry. Van's contributions go way back. He had the kind of mind that would think of a total concept, the music, the dance. He was as good as gold, and that also bothers me, that the three of them were so young."

Her career was closely tied with Donny Hathaway; their union earned a Grammy award in 1972 and a nomination this year for "The Closer I Get to You." On the day of his death, the duo had been in the studio, working on two songs Stevie Wonder wrote for them, and had had dinner together.

After two months away from it, Flack went back to work on the album and added two songs of her own, and the finished product will be released in the fall.

"Beyond the emotional thing, just returning to singing and performing was hard. I had to get back into the studio but at times I had to leave. I was too distraught. And I had to discipline myself because it was too expensive to cry," said Flack. The cost of studio rental, muscians, and technicans, she notes, improved her discipline. But she still has some memories that make her glow. And it's not the Grammys, nor her sprawling aparment in New York's Dakota building, nor the People magazine spreads.

"I think the highlight of the last 10 years was performing in Israel last August," she said. "We played six concerts in Tel Aviv, Jerusalem, Haifa and Caesarea. In Caesarea the people were fabulous, they wouldn't let us off the stage. After we finished, they applauded for 25 minutes. I had to put my clothes back on, slap on some lipstick. They were screaming for "Bridge Over Troubled Waters," which I had not performed in three years. But the words came back."

On her list of professional low points, passing up a chance to record "You Light Up My Life" ranks first. "It was too cliched, it reminded me of ballads I had done.

"And so, 11 million singles later..." recalled Flack, laughing.

Still, she remains a star, but an insecure one on what was once native ground. "Things happen, just the distance, that make you seem remote," said Flack. "It's awfully hard to come home again. It's just different when many of the wonderful things that happen to you happen away. You don't know if people appreceiate the differences or if they misunderstand them and see you as Cinderella."

Once on stage, with the full moon providing a majestic lighting, Flack gave her familiar toss of the head, her full cheeks parallel to the sky. For those who thought she had gone too soft and moody in recent years, she delivered the old Flack style of preaching and wooing. Several times she said, "it's so good to have 175,000 of you out there."

When her return Washington concert was all over, having survived a generator failure, about 100 people gathered around her Winnebago at the south end of the Capitol grounds. From inside the Winnebago, Flack signed autographs. Watching was her mother, Irene Flack, who gave the first critique of the evening, calling her daughter's voice "more mellow," before she stepped into a waiting limousine. CAPTION: Picture, Roberta Flack, by James M. Thresher - The Washington Post