The Berkshire Museum wouldn’t need to sell some of its most prized works of art to remain operational if it grew its endowment by $4.5 million, according to an expert on nonprofits. “I continue to believe that they have overstated how much of an emergency they are in,” said Stephen C. Sheppard, “and I think there would exist a clear path to a sustainable, essentially status quo, outcome that would not require deaccession of the 40 artworks.”