K-tuples are not difficult to calculate from the databases, I have a perl script
for doing this from the EMBL database which I can make available to you via e-mail.
Alternatively, look in Methods in Enzymology, there is a volume dedicated to
analysis of DNA sequences with a paper by JM Claverie et al, which makes reference
4 and 6-tuple occurences.
saparici at med.cam.ac.uk
Molecular Genetics Unit
Phone +44 223 402406
Fax +44 223 210136