It's only redundant because you're ignoring the Astral Fire (which keeps Enochian up, by the way) and the Firestarter proc Fire grants, whereas Fire IV is just a single target spell that does massive damage (260 potency in SB + Astral Fire III damage buff) you can only use while under the effects of Enochian and Astral Fire (of which it does not keep up).
Both of which could easily be incorporated into Fire 4's effect, since both ways you're still going for the same thing -- damage on a single target. That's pretty much what I've been saying all along. Keep up.