(Usually) in positions with the pair of bishops, pawns are placed on squares of different colour to the bishop that has been left without a direct opponent, taking away squares from the knight. Luis Bernal, The Berlin Defence Unraveled
Often the weakness of an advanced pawn is not so much its liability to be attacked, but in the loss of control in a neighbouring file. Colin Crouch & Kean Haines, Hastings 1895
4.Bd2 (in the Nimzo-Indian) has been heavily slagged off in the popular press. The criticism runs along the lines of "this move is just too passive." My opinion is that any move which serves a useful purpose (in this case unpinning) must be treated with some respect. John Emms, Easy Guide To The Nimzo-Indian
The Scotch Four Knights is an ideal choice for the improving player, whether playing White or Black. The variations are neither too complex, nor theoretical, both sides develop easily, and the open positions are ideal for honing tactical skills. John Emms, Starting Out: The Scotch Game