A Brooklyn-based property owner has filed applications for a four-story, three-unit mixed-use building at 716 57th Street, on the southern end of Sunset Park. The project will measure 5,856 square feet and rise 42 feet to its roof. There will be 1,080 square feet of medical office space on the ground floor, followed one apartment per floor on the second through fourth floors. The apartments should average 975 square feet apiece, which means either condominiums or rentals could be in the works. Luis C. Wong’s Flushing-based engineering firm is the applicant of record. The 20-foot-wide, 2,003-square-foot lot is vacant.