A group of investors led by Kenneth Hirsch’s Gotham Capital Realty just picked up a small Downtown Brooklyn office building with big development potential. The buyers paid $46.6 million for the four-story, 28,000-square foot office property at 68 Willoughby Street, according to property records filed with the city Thursday. The building, constructed in 1920, contains four office units and a group-floor retail space. It has a total of nearly 120,000 buildable square feet, records show.