It might be hard to be a second fiddle, but if the Primavera Quartet is any guide, it's even harder to keep one. The all-woman string quartet, which will perform this Friday and Saturday at The Wolf Trap Brahms Festival, has had four second violinists in the last seven years.

"It just happens to be the hot seat," said first violinist Martha Caplin, who has played with violist Diann Jezurski and cellist Melissa Meell since 1975. "It's the old story. You have to play as well as the first violinist, but you very rarely get the same kind of attention."

The young musicians, who won the prestigious Naumburg chamber music prize in 1977, were the subject and stars of a musical last year by librettist Tina Howe and composer Stanley Silverman, Caplin's husband. "The Columbine String Quartet Tonight!"--as it was called--treated the troubles and adventures of a budding all-woman quartet.

But last August, the show fell victim to its own troubles and adventures. After it was booked into Canada's Stratford Festival for a three-week run, second fiddle player Mitsura Tsubota, the third woman to hold that chair, decided to quit and get married in Las Vegas. That stopped the show and whittled the quartet to a trio. The group's frantic, month-long search produced second fiddle player Catharine Metz, a freelance musician living in Philadelphia.

"I think my personality is well suited to the job of playing second violin," Metz said the other day from Montana, where the group had just given a concert. "I've had to learn a lot of repertoire in the last five months, but I feel very comfortable under the circumstances. I don't have a problem with that old cliche' about 'second fiddle' being a derogatory term."