Ludwig von Beethoven’s birthday is next week. Musicians all over the world are marking his 250th year with celebratory performances. Constrained by the pandemic, the Sarasota Orchestra is celebrating with a compact, meaningful concert of two of the master’s string quartets. The program opened Thursday night with his String Quartet No. 4, Op. 18 in C minor from Beethoven’s first set of quartets composed while still bearing the shadow of his teacher, Franz Josef Haydn. Pioneer of the form, Haydn had set a high bar for standards in the classical style for all to follow. Beethoven, however, was already thumbing his nose to classical style, though stealthily at first. Seemingly smooth and melodically tame, syncopated accents and unexpected harmonic shifts catch the listener off guard in this early quartet.