When last we visited the lonely blue-glass office building next to the former site of the River Oaks Hospital, just west of Greenway Plaza, the brand-new structure had only a single tenant — a doctor. That was more than 2 and a half years ago. But Cushman & Wakefield’s latest flyer for River Oaks Plaza at 4140 Southwest Fwy. (it’s been on the market ever since) lists the 5-story, 105,000-sq.-ft. building as “fully leased.” Was there a huge influx of tenants in the meantime? Not exactly, though a second doctor’s office did move into the building a bit later. In fact, both offices are leaving the building. That’s so the Art Institute of Houston can move in, beginning in July. The institute and its 2,300 students will relocate from this somewhat less see-through 6-story building at 1900 Yorktown, in the Galleria area: