The Washington Opera yesterday announced four operas to be presented in its 1979-80 season. They include the company's first "Tristan und Isolde" by Wagner, with Bayreuth stars Roberta Knie and Spas Wenkoff in the title roles.

The season will open Sept. 15 with Massenet's "Cendrillon," which is the Cinderella story in romantic French style. The cast will include Frederica von Stade in the title role with Maureen Forrester, Ruth Welting and John Reardon. While the company did not name a conductor for the Massenet opera, it is expected that Julius Rudel, who recorded it in Europe last summer with almost the same cast, will be in charge.

Verdi's "La Traviata," with Catherine Malfitano, Beniamino Prior and Brent Ellis as principals, will also be heard in September with Theo Alcantara conducting.

"Lucia di Lammermoor" by Donizetti will be given in February 1980, with a cast headed by Ashley Putnam, the star of the company's recent "Abduction from the Seraglio." Putnam will be joined by Romanian tenor Vasile Moldoveanu and baritone Juan Pons.

The "Tristan" will be the fourth of the operas in the season. In addition to Knie and Wenkoff, the cast will introduce to Washington mezzo Hanna Schwarz as Brangaene, Victor Braun as Kurvenal and William Wildermann as King Marke. Although the opera company did not name him, it is understood that Michael Tilson Thomas will conduct the Wagner score.

The company is announcing four performances of each opera but is hoping that it will be possible to present each one five times with the exception of "Tristan."