Hopeknight, when the 4th did the jutsu, the soul of the person he was trying to steal was consumed by the death god, kyuubi's soul isnt consumed, its not exactly the same then. and if its not exactly the same then maybe the soul of 4th wasnt consumed....
Since Kyuubi is a Youma and supposed to be immortal, than it couldn't be consumed by the death god. It that sense, the jutsu would be useless against Kyuubi, so the 4th would've made an adjustment to the seal so that the Kyuubi would be sealed into Naruto instead of himself.

The way the original jutsu works is that the user seals the target within himself at the cost of himself being devoured by the death god. So the death god consumed the 4ths soul for using the jutsu, however, the kyuubi was not placed in the 4th, but within Naruto.

The 4th is dead. Fact, not assumption.