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If fourth is victory, and second is defeat, is the third line the line of the stalemate?
I think that calling the fourth line the line of victory is fairly recent. Calling the second line the line of defeat goes back many years and refers to the fact that crawling along the second line only adds one point of territory (unless it is necessary for some other reason). So if your opponent can force you down to the second line, that is usually bad for you.
Perhaps people thought, if there is a line of defeat, isn't there a line of victory? Well if your are forced to crawl along the fourth line, that's usually good, so let's call the fourth line the line of victory.
The third line is the line of reasonability, I would say :-)
I generally call it the line of territory as do many other people. The person on the third line is generally getting the same amount of territorial worth as the person on the fourth line getting influence.
Actually, according to Minue, this is a misunderstanding of conventional wisdom in Asia, fed by the widespread though disputable classical diagram.
As he told me, the conventional wisdom must be understood as: the third line is the line of stability, that is: acquiring eyespace which cannot be easily undermined. Thàt is the true nature of the third line, which is true on any board size, not just 19x19.
The fourth line in its turn is the line of development, that is, it is efficient to develop along this line, because, if the opponent undermines your stability and makes a stable group himself, you'll be able to press him down from the fourth line and acquire powerful influence over the rest of the board. This too is true on any board size.
How to combine plays along these lines is a question to be answered more subtly than just showing the territories arising from such play. One can indeed imagine 4th line influence to be worth more on a 21x21 but also to be worth less on a 31x31.