Black should resign if one player has four corners
The proverb "Black should resign if one player has four corners" is a combination of two proverbs: "If Black has all four corners, Black should resign" and "if White has all four corners, Black should resign".
Therefore, if White has managed to capture all four corners, it is assumed that Black has lost the game. Conversely, if White has let Black take all four corners, White has taken so much profit elsewhere that Black should still resign.
-- Morten Pahle (10kyu)
See also Proverbs Do Not Apply To White.
This proverb seems to be the weakest one. Let's consider a 9 handicap stones game (or any >4 handicap stones game), the 4 corners are open. This proverb encourages beginners to protect their corners in handicap game even if it's a bad idea (because hoshi isn't conceived for territory).
Besides, if white player invades the 4 sansan, black might want to resign (because of the proverb) despite the fact that he's probably ahead, as shown on this diagram.Guillaume Chanson?
Don't take it too seriously. In fact don't take it at all seriously. Charles Matthews
kevinwm: But this example misses the point entirely. In a 9 stone game, no competent white player would allow this situation to occur. Sure, he might start by taking one corner, but would never allow b to create the tremendous center shown here. If w did manage to get all four corners, it would probably be in addition to a nice sized living group in the middle....
se: I have to second kevinwm. What proverb is accurate every time? The point is that in most games, especially amateur games, most proverbs will hold, and are worth keeping in mind. This proverb reminds black not to overvalue the corners.
See also the alternative (is it maybe older): IfYouHaveLostFourCornersResign
I will point out WuRenGo2.