No.4 and Attached Railings

House, now flats, completed in 1790. It was designed by John Palmer and built by William Hewlett. The front of the building is limestone ashlar, painted to the ground floor, with a rubble basement and rear elevations. The roof is a double pile mansard roof covered in Welsh slate, featuring a coped party wall to the left and two rebuilt ashlar stacks. A staircase is located at the rear.

The house is three storeys, attic and basement, with a three-window front. The first floor has three grouped plate glass sash windows with horns, narrower on the left and right sides, set in plain reveals. The second floor has similar sash windows, with four-pane over four-pane glazing on the outer windows and six-pane over six-pane glazing on the central window. The ground floor has two plate glass sash windows with horns, also in plain reveals, and a six-panel door with fielded and glazed panels. A pennant paved crossover with a cast iron footscraper is flush with the pavement. The basement has two sash windows with six-pane glazing. A plank door is set into an ashlar infilling beneath the crossover, with twentieth-century area steps. There is a triple dormer with a plate glass sash window. Architectural details include a band course above the ground floor, a weathered sill band to the first floor, a frieze, a moulded eaves cornice, and a coped parapet. The rear elevation incorporates early six-pane sash windows on each floor, along with nineteenth- and twentieth-century ashlar, glazed, and rendered extensions to the second-floor landing, including a nineteenth-century half-glazed door with coloured and etched sashes. The interior was not inspected during the listing process.

Attached wrought iron railings and a gate feature cast urn heads on limestone bases. This property was part of an incomplete development of St James’s Square, built on land leased from Sir Peter Rivers Gay. Marlborough Street forms one of the four diagonal approaches to St James's Square. An underlease was granted to William Hewlett for 96 years in 1790.