This is completely my invention but I think that the number of tails is related to what Naruto is able of. In the begining it was stated by Jiraya, if I correctly recall, that Naruto couldnt take out much of the kyubii chakra because he was "a too small container". I understand that in the way that naruto wasnt able to gather much chakra or release it.
As time went by he gained more stamina, more chakra and more control over it letting him release higher amounts of kyubii chakra.
In the begining against Haku: a huge amount of chakra but no tails (and that was the time that naruto has been most pissed imo).
In the middle: Against Neji he released the chakra in the same way. And against Gara too but here he was releasing way more chakra than in previus fights.
Then against Sasuke he released one tail and lots more of chakra.
Now he is able of releasing 4 tails (but only 3 effectively).
So the stronger he is the more tails he is able to release. Consecuently more tails-> less control over it.Tthus more chances to go into berserk.
After all this nonsense I have to say that I would be very dissapointed if after hearing the speech from Yamato about Naruto using his own strength and not relying completely to kyubii fast power boost he just gets a technique from Kakashi to use it even more.
This is the similarity and difference between Naruto and Sasuke both have a cheap way of becoming strong but Naruto has shown that he doesnt take shortcuts. Because "the arent shortcuts to become Hokage".
I hope they let kyubii rest for a bit, even if they use his chakra I dont want any more tails!!