Once again, due to the lack of outstanding peaks and valleys in the letter frequency, it is highly unlikely that K4 is a uniliteral transposition or substitution cipher. That being said, I could very well be wrong and the text choice by Sanborn throws off statistical analysis. In the spirit of not being sure, I’m archiving some letter strings (without the apparent KRYPTOS letters) of the 90 remaining letters in case they should prove useful at some future point.

1×90

OBUOOXGHULBSOLIFBBWFLRVQQPRNGKSWTQSJQSSEKZZWATJKLUDIAWINFBNVTTMZFPKWGDKZXTJCDIGKUHUAUEKCAR

2×45

OBUOOXGHULBSOLIFBBWFLRVQQPRNGKSWTQSJQSSEKZZWA
TJKLUDIAWINFBNVTTMZFPKWGDKZXTJCDIGKUHUAUEKCAR

3×30

OBUOOXGHULBSOLIFBBWFLRVQQPRNGK
SWTQSJQSSEKZZWATJKLUDIAWINFBNV
TTMZFPKWGDKZXTJCDIGKUHUAUEKCAR

5×18

OBUOOXGHULBSOLIFBB
WFLRVQQPRNGKSWTQSJ
QSSEKZZWATJKLUDIAW
INFBNVTTMZFPKWGDKZ
XTJCDIGKUHUAUEKCAR

6×15

OBUOOXGHULBSOLI
FBBWFLRVQQPRNGK
SWTQSJQSSEKZZWA
TJKLUDIAWINFBNV
TTMZFPKWGDKZXTJ
CDIGKUHUAUEKCAR

9×10

OBUOOXGHUL
BSOLIFBBWF
LRVQQPRNGK
SWTQSJQSSE
KZZWATJKLU
DIAWINFBNV
TTMZFPKWGD
KZXTJCDIGK
UHUAUEKCAR

Enjoy!

–Kryptos Fan