Sanborn, 2006, on the K2 correction: the answers to the first three passages contain clues to the fourth. Sanborn, 2005: "I have left instructions in the earlier text that refer to later text." Sanborn, 2009 oral history: K4 "will come back to the first part". Counterweight, Sanborn to Schmeh, 2019: it is not necessary to know K1-K3 to solve K4. Read together: K1-K3 supply the key material, not the method.
- Words and phrases from the K1-K3 plaintexts, especially the anomalies: IQLUSION, UNDERGRUUND, DESPARATLY, LAYERTWO, PALIMPSEST, ABSCISSA, the K2 coordinates as digits, WW, "can you see anything".
- The sculpture's own text: KRYPTOS, the Morse fragments, the HILL column, the raised YAR.
- Sanborn's named themes: BERLIN, CLOCK, WELTZEITUHR, EGYPT, CARTER, the 1989 wall.
- Only then generic English word lists.
Every experiment's bounds should say which key source it drew from.