Despite its slim footprint, the five-bed terraced house has an underground double reception room, cloakroom and spa, containing a sauna, jacuzzi and large shower. This is connected to the upper floors by a lift and a spiral staircase.
The ground floor hosts a modern, open-plan kitchen diner, utility room and garage, which is accessible from the street.
The five bedrooms are all on the upper floors, with one currently used as a reception room, and two with ensuite.
The property on Catherine Place, close to Buckingham Palace Gardens, also has a roof terrace and a private garden.
According to Dexters, the property’s width is an advantage in terms of bringing in natural light.
On each floor, for example, there are two strategically placed sash windows, which allow daylight to enter the full depth of the house.
The estate agent also said that having a narrow home can help to reduce energy consumption in heating or cooling the rooms.
Harry Laflin, manager at Dexters Westminster, said: “If you like traditional properties that tick all the boxes, then chances are this space is not for you,” says Harry Laflin, manager at Dexters Westminster.
“If, however, you don’t like boxes and you like to entertain, enjoy the quirky and charming and feel there is more to life than two-up two-down, then come and see a truly special home."
The skinny home was last sold in 2005 for £1,200,000 – a third of its current asking price.
Westminster has the highest property prices of any borough in London, according to Rightmove, with terraced houses selling for an average of £3.5 million in the last year.
At £4 million, therefore, this property is more expensive than most, despite its four-metre width.