Every now and then, it’s appealing to throw in a piano to the already perfect string quartet lineup of two violins, viola and cello. The Spokane String Quartet is doing just that Sunday inviting pianist Lydia Brown to the stage. And Brown has the credentials to impress — not only has she earned a doctorate from Juilliard, she also holds degrees from Eastman and Yale. Currently, Brown serves as assistant conductor at the Metropolitan Opera. The program will include a piano quintet by Brahms and piano quartet by Mozart.