Val,
It makes good sense to me.
We're seeing the same problem of repetition even earlier and even with Singapore Math, which is supposed to be one of the good ones for being linear rather than a spiral curriculum. I've actually been considering moving on to pre-algebra or some more complex geometry with my DS6 while I wait for him to learn his times tables. That seems so bass-ackwards that I've been dithering over it for some time now. But for him, it seems to make sense.
If you have the grades 5-8 books, you can always use them to supplement any holes that pop up as you're doing the algebra. As long as he doesn't get so confused that you have to un-teach him what he's learned wrong, I don't see the harm that could come. It seems to me that in the worst case, where it is too hard or leaves him with too many holes, you go back a grade or two. <shrug> No harm done.
K-
