I'm confused but that's probably me being thick:
I thought that in TEH it was the only confirmed scene in Mycroft's office. But in the tumblr exchange there's talk of a room with yellow walls?
Secondly the room at the end of THoB, I always thought that was not an office but some room for inquiries with prisoners or something. How do we know it's Mycroft's office as well?
And what allusion is there to this wooden horse tweet??
Sorry for those dumb questions but I'm curious and eager to understand....

The horse referred to a recent tweet by Arwel, it might be in the series 4 thread. 

As for the rest: 
The idea is that cell/interrogation room we see at the end of HoB is quite similar to Mycroft's office in TEH. We see the office again in HLV - in Sherlock's mind palace and when Lestrade meets Mycroft to discuss the boltholes. In TGG, however, Mycroft had a completely different office, a bit old-fashioned and not in a basement either. You can see it when John meets him there. 

The rooom in HoB is very naked, concrete walls and floor. Take a look at Mycroft's office in series 3, same naked walls and floor. Quite unusual for a man of his standing. 

Then there are these strange round indentations in the walls, both in HoB and series 3, which look quite similar. We were thinking what it could mean if Moriarty was held in a room looking like the one Mycroft uses as his office. And why he changed his office at all. 

I will be disappointed if we don't get more information on Mycroft in S4.  Specifically what he did or did not know about Mary. 

We are shown that Sherlock is Mycroft's one pressure point and he knows that Sherlock's main pp is John.  Mycroft mentions keeping a casual eye on John during the Hiatis but knows without consulting anything where and when John will be having dinner that night.  You can not tell me he knew nothing about who John was involved with.

We've been told that S4 is going to be about consequences.  I believe that some of those consequences will be for Mycroft.  After the events of HLV I think some of the games Mycroft has been playing since S1 are going to come to a shattering conclusion.

To that end I absolutely think that the set design and lighting choices in Mycroft's office could be seen as foreshadowing of a kind.

SusiGo wrote:

I wrote a little meta which I post here because it refers to (at least) series 3 and TAB. I had a closer look at the second modern scene in TAB and came up with this:

http://gosherlocked.tumblr.com/post/143165439481/hlv-and-tab-the-hospital-clue
 

Interesting ideas there Susi!