The reason why I thought he went four tails and lost control was because he voluntarily went into the gate. Every other time he got chakra from the Kyuubi he was always outside the gate. But when he went in thats when he lost control. So I think that was symbolic about how he was willing to do anything to get Sasuke back even let evil take him over. I believe Naruto knows going into the gate is bad, but he did it because he believed it would help him get Sasuke back. Thats just my opinion i might be wrong.