The St. Lawrence String Quartet estimates they've given about 2,000 concerts in their 20 years together. They never hold back in performance, and every chance to see them is special, although, thankfully, not rare. This weekend the group anchors the final Sundays@Four concert of the season at the Crowden Music Center in Berkeley. On the program is Beethoven's marvelously lyrical Quartet in C-Sharp Minor, Op. 131, Haydn's Op. 20/4, and Canadian composer R. Murray Schafer's String Quartet No. 3.