At the corner of 51st and Fourth, the Parkway Garage (1907). I'm not sure how the whole building is used today, but one of the businesses in residence is Triple Tex Fabrics Inc.
The building's a reminder of a statelier Fourth Avenue.
In the 1890s, Fourth Avenue was planned as a “parkway” to South Brooklyn, with planted malls down its center, much like Park Avenue in Manhattan. Unfortunately, the malls were demolished when the subway arrived in Sunset Park in 1915 to make way for vents. Until the 1950s, when car ownership and vehicular traffic were on the rise, Fourth Avenue’s sidewalks were roughly eight feet wider than they are today, to accommodate the popular 19th and early 20th century pastime of “promenading.” (Six to Celebrate: Sunset Park - Historic Districts Council)