In Game of Thrones (second edition):
I have marveled at the balancing put into this game, specifically for 3-player games, where the whole south of the map is blocked, forcing the game to stay in the north, and thus creating a very good balance between the 3-players.
The 5-player variant is also very well balanced. Including all of the map in the game with no blocked territories, the only difference is that house Martell is gone which releases some tension on the board so the game is simply delayed a bit, but still very balanced (IMHO).
And the 6-player version (the recommended) is obviously balanced, as it is the main design of the game.
However, I feel that the 4-player version of the game is extremely unbalanced compared to the other variants.
First of all, there are no blocked territories, creating a huge empty space in the south, the only thing to hinder a rush are neutral troops which can be easily defeated after the first few turns.
But worse still is that house Tyrell's starting spot (Highgarden) is left completely unguarded, almost requiring house Lannister to move in.
House Baratheon is almost unhindered in its capture of all of the south unless Stark moves in aggressively or Baratheon moves inland or north, which barely happens when the south is so seductive.
House Lannister can also grab for the south, but much keep care of Greyjoy as they have easy access to the lands as well. As long as Lannister leaves the Riverlands and goes south, they are pretty safe from Greyjoy for the start of the game.
House Greyjoy, the extra house which makes the difference from 3 to 4 players, is stuck between Lannister and Stark, but with the empty south, and both Baratheon and Lannister moving down the board, Greyjoy is pretty free to take the Riverlands, unless Stark moves in, which would weaken both players and block their progress, and let Lannister or Baratheon get a big lead at the start.
House Stark has a great time of this board, being able to see what the other players are doing, and where they are focusing, and then adapting. Being able to go either direction on the board, and also into the Riverlands, they are in a great situation on the 4-player board and it is very difficult to do anything to them.
If Baratheon moves against Stark, Lannister or Greyjoy can easily grab the Riverlands, or the south, if Greyjoy moves to the Riverlands and north then Lannister can land-grab in the Riverlands or move south with ease.
The above are just a few examples of some of the things I feel are wrong in the balance of the 4-player game.
I would like to know if others have suggestions on how to improve the balance with house-rules, or if I am overlooking something, to please inform me as to how it actually is balanced.