Considering they said going Kyuubi causes damage to his body I imagine it must be pretty painful, and more so with each new tail, because the transformation into a demon form seems to increase step by step.

So, after the cause of anger has vanished, I reckon it'll happen like DE said: The anger will subside and he will head for a less painful state, that is, back towards the human form. Though thinking about the bigger picture there could be a peak value somewhere after which he can't return unless some exterior force encourages him to, but if there is such a peak I suspects it's closer to nine and not so low as four.