We use this program at our school--check out their website because I know for first grade, they give you a list of the spelling words along with challenge words that follow the same concept (like short e). Maybe you could look at the fourth grade area---I haven't checked out any other grades though.
I have a fantastic speller in my class this year--the best I've ever had and no challenge lists seem to be difficult enough for him. Instead, I've decided to focus on word study--how words work and what they mean. I've done lists for certain prefixes, suffixes and their meanings. Also how the root words may change when adding suffixes (double consonant, drop e, etc.). When we were studying continents, I gave him theme-related words (all the continents, oceans, etc.). I've been using the Words Their Way sorting books to guide the order of what I introduce and to give him sorts. It takes some extra work each week but he and his parents really appreciate what he is learning.
We use the advanced word list from Scott Foresman Reading Street. However, the kiddos are to study the word meaning as homework then when we test them on Fridays, we give them the definition of the word and a sentence with the word in it but leave out the actual word. The students are to write the correct word without hearing it. Quite challenging. And good vocabulary work.