4 Impalers would scare me, but you're units would have to be pretty thin to support those four Individual slots right? That is, of course, if I'm remembering right that they are individuals. Anyway, you would certainly get some good target practice in on the big chicksta's. The problem with four Impalers, in my opinion, is that it's less versatile than 2 Soul Flayers in a tournament setting. The two big ladies are just all around bad-ass and I think are pretty adaptable. Against a heavy infantry force I think that the Impalers lose some effectiveness.
Mark