Quote Originally Posted by Abdula
I also think that there is some other reason why Naruto is suddenly able to use that much of the Kyuubi's power, anger alone can't be the trigger. If you think about it, the angriest we've seen Naruto's was when he was trying to stop Sasuke from leaving the village. If all he needed to be able to use so much of the Kyuubi's power was anger then we would have seen more than one tail at that point, so I think there is something else that dictates the number of tails that are formed.

I also find it very interesting that Naruto went four tails against Jiraiya, again if his anger is the trigger what could Jiraiya have ever done to make Naruto that angry, when they were just training. To go four tails Naruto has to become completely irrational, even against Oro and Kabuto they had to taunt and provoke him just for him to go that far and he already hated them so much.

So there is another reason why Naruto can only go four tails now and I suppose whatever that reason is, it is also why he wasn't able to go beyond one before.
Jiraiya said the Fourth's seal is weakening. He probably can't go beyond four tails now because the seal doesn't allow it to. Before, two tails was max. That's why they anticipate eventually nine tails will form.