I think Sitton Spelling, used in our school, is too easy in general and certainly too easy for my 4th-grader son. Here is a list of Sitton 4th grade spelling words:
http://www.johnston.k12.nc.us/education/components/docmgr/default.php?sectiondetailid=100145& . One criticism of Sitton Spelling is at http://www.nrrf.org/essay_Elam_on_Sitton.html .

What is a good, challenging spelling program? I suppose spelling and vocabulary should be integrated.

"To see what is in front of one's nose needs a constant struggle." - George Orwell