K4 is a key. It is in fact, in the shape of a key. But it is not just a common key. It is a BERLIN key. (Look up the explanation). The doorway that it fits into is the original K2 matrix. Using David Stein’s original matrix, you’ll see a unique letter pattern in K2 that matches a letter pattern in K4. Matching up these patterns gives you the unlocked position.
CLOCK is Sanborn’s wordplay on C LOCK or SEE LOCK. Using the clue from K2 (WW), you can find another matching word pattern on K4 and K2 which will give you the LOCKed position. Sanborn is saying that this locked poisiton is the one you should be using. Then it is matter of using several clues found throughout various parts of Kryptos to find the message.
