Lenny, thanks for your question.
Directly, text comparison works between different texts of the same passage, rather than different passages in the same text. However, you can do some sort of limited text comparison using the HITS command. For example, you can set up one tab with the search * [RANGE Song 4:1-7], then another tab [RANGE Song 6:4-10] *@-[HITS Tab1]. (where Tab1 is the name of your first tab). This will show you all lemmas used in Song 6:4-10 that aren't used in Song 4:1-7. You can do a similar thing, comparing the inflected forms (using HITS=i) or the tags (using HITS=t) to find your differences.
I don't believe there is any way to do this involving roots, and as you will tell, the searches also are not respecting the word order of each verse, but it may help you get closer to what you are looking for.