I can't figure out how to post in the "Community Releases" board, but this seems relevant as well.

Seeing a screen shot here:

http://theportalwiki.com/wiki/File:Port ... ent_02.jpg
which has a different observation room from the one featured in the editor, it inspired me to clone the observation room and create a 4-block wide observation room.
It is not identical to that screenshot, but it has similar principle.

Download:

http://bisqwit.iki.fi/kala/ptal2/bisqwi ... 12x256.vmf

http://bisqwit.iki.fi/kala/ptal2/bisqwi ... simple.vmf
Save into "Portal 2/sdk_content/maps/instances/p2editor/"
The difference between these two is that the "simple" version omits some decorative objects (such as boxed turrets) from the room.

Screenshot:

To use in puzzlemaker, add this to your editoritems.txt:

   "Item"
   {
      "Type"      "ITEM_OBSERVATION_ROOM_4"
      "Editor"
      {
         "SubType"
         {
            "Name"         "4x1 observation room"
            "Model"
            {
               "ModelName"      "observationroom.3ds"
               "TextureName"      "observationroom.png"
            }
            "Palette"
            {
               "Tooltip"   "4x1 observation room"
               "Image"      "palette/observation_room.png"
               "Position"   "3 2 0"
            }
         }
         "MovementHandle"      "HANDLE_NONE"
         "InvalidSurface"      "CEILING FLOOR"
         "DesiredFacing"         "DESIRES_UP"
      }
      "Exporting"
      {
         "Instances"
         {
            "0"
            {
               "Name"            "instances/p2editor/bisqwit_observation_room_512x256.vmf"
               "EntityCount"      "33"
               "BrushCount"      "15"
               "BrushSideCount"   "119"
            }
         }

         "TargetName"      "observationroom_huge"
         "Offset"      "64 64 64"
         "OccupiedVoxels"
         {
            "Voxel"
            {
               "Pos"      "0 0 0"
               "Surface"
               {
                  "Normal"   "0 0 1"
               }
            }
            "Voxel"
            {
               "Pos"      "0 1 0"
               "Surface"
               {
                  "Normal"   "0 0 1"
               }
            }
            "Voxel"
            {
               "Pos"      "0 -1 0"
               "Surface"
               {
                  "Normal"   "0 0 1"
               }
            }
            "Voxel"
            {
               "Pos"      "0 -2 0"
               "Surface"
               {
                  "Normal"   "0 0 1"
               }
            }
         }
         "EmbeddedVoxels"
         {
            "Volume"
            {
               "Pos1"   "0 -2 0"
               "Pos2"   "0 1 -1"
            }
         }
      }
   }

And edit the "Palette" section as necessary.
The icon for the room is still that of the regular observation room (in-puzzle, 2x1 room), because I haven't learned yet how to convert an image into a VTF file, nor how to create 3DS files.

Note: Because the spotlight in this room is farther away from the window than in the standard observation rooms, the spotlight beam is vertically much narrower than in those others. The bottom edge of the beam descends at a rate of about 26 pixels vertically per 64 pixels horizontally. The top edge of the beam ascends at about 10 pixels vertically per 64 pixels horizontally.

You can create VTF files with the tool VTFEdit, which also lets you edit existing ones.

The 3DS file is just a selection mesh, you need to create an MDL file. It's more complicated than creating a VTF, and I'm not good at it, so I'll leave it for someone else to explain.

I also updated it a bit: The observation room was somewhat darker than it should have been, so I cranked up the light a bit. It is still not as bright as the small observation room (that oddly has no light fixture props at all), but it is in the same realm now.
I also removed the player clip element from the room. If the most resourceful player figures out a way to glitch themselves into the room, so be it.

There is slight texture fighting on the right-side part of the room, near the window inside edges. I don't know what is causing it: As far as I see in Hammer, that part is a perfect clone coordinatewise from the other part.