"Opulent, luxerious, expensive, thoroughly decadent."
That's Olgas first impression of John Steed's fourth flat in The Avengers  in The Correct Way to Kill.
The 4th FlatHomeSketch of 3 Stable Mews
There is guest room's door on the right of the windows. There are two entrances to the apartment, one over the stairway and the corridor, the second through a side entrance, which is next to the kitchen. A spiral staircase leads to the upper floor where the bedroom, the bathroom, and other rooms should be.
The walls of the living room are fully equipped with wood paneling, which probably consists of light oak, and the flat's ceiling is painted white. A large double window opens onto the street Stable Mews. The floor-length window curtains are yellow. In front of the window stands the three seater Chesterfield sofa typically. The living room furniture also includes two large red armchairs, which do not have the same color as the Chesterfield. On the right in the living room stands Steed's desk. He's using different desk chairs (at least three).
From the living room lead two steps to the kitchen and the dining room area.
There is also a spiral staircase that leads to the upper floor. A low wall separation divides the kitchen from a dining area. The kitchen is painted in blue and light brown. Steed is using it as a darkroom in From Venus with Love. The dining area is painted flaxen. Steed never used it neither during the Emma Peel era nor season 6. When he's having dinner, such as in Get-a-Way! or Pandora, he's using a small round table in the living room.
Is Steed taking souvenirs? The same painting can be seen in The Danger Makers.
The same landscape painting above Steed's mantlepiece can be seen in the auction room in Have Guns - Will Haggle.
I doubt that anyone except Steed would like to live there as this is the "Death Flat."

At least sixteen people died in 3 Stable Mews, but to Steed's defense: it's not his fault. Most people who died in his flat got killed by foreign agents or rivals. There are three deaths in Legacy of Death, one dead in Killer, one in False Witness, nine in Who's Who??? and there's one corpse in A Funny Thing Happened on the Way to the Station. The first dead man in Steed's flat is Tubby Vincent, a colleague of Steed in Escape in Time.
But it's no wonder that some many people got killed there because almost anyone - friends and foe- seems to know the address of the flat. And apparently it's quite easy to break in:  Prendergast (The Joker) planted a trap, Benson swapped Steed's watch (The Return of the Cybernauts), Gordon placed a bomb in Something Nasty in the Nursery.
The door is secured by a bolt and a chain, but often Steed left the flat door open.

But there are more pleasant records too (maybe not for Steed's liver).
It is the flat where the most bottles of Champagne got emptied. Flat 2 got the second place, followed by no. 3 and Steed's Stud, while Steed never had a bottle of Champagne in Flat 1.
John Steed's Flats - No.4
3 Stable Mews
Only in one episode, A Funny Thing Happened on the Way to the Station, Mrs. Peel ever used a key to Steed's fourth flat.

There is a hidden door release on the top right corner of the door (Who's Who???)

It's the only flat of Steed from series 1 - 6 with a televison, which can be seen twice (The Morning After and Pandora).
Steed's fourth flat consists of two floors. On the first level, there is a kitchen, a semi-circular dining area, a large living room. The living room opens to a storage room, that "disappears" after Who's Who??? and was not existing before the episode, but maybe the door is simply covered with oak panels.
But that wall is essential as the "fourth wall" and they rarely did reverse shots in The Avengers.
There is also an extensive collection of militaria in his flat, e.g., Long firearms, models of cannons, drawings of soldiers, horns. Above the fireplace is a large painting, The Charge of the Light Brigade in the Battle of Balaclava on 25 October 1854 from the Crimean War painted by Robert Alexander Hillingford in 1899 (the same painting can be seen in The Danger Makers). Since the Tara King series, it's replaced by a landscape painting of an unknown artist (it's the second landscape painting in Steed's flat, as he had another once hanging right to his entrance door in flat 3).
For a man who despises the use of weapons Steed has a pervasive collection. In the desk drawer, he keeps a revolver, a Smith, and Wesson.