It's easy to make the mistake of thinking Fire II always beats Fire IV on 3 targets just by looking at potency, but there are other things to consider like MP cost and how much HP the mobs have.

Fire IV rotation with a Fire II to refresh AF3, spreading Thunderclouds and ending with Flare is pretty powerful if the 3 mobs have enough HP. Overall I'd say it's safer too, if you miss one mob or one dies before Fire II goes off then it's a DPS loss.