Dont know if its been said in some other thread but IF infact the Kyubi is imortal then if the person its sealed in dies then it will be released so if the 4th had sealed it into himself when the death god killed the 4th the Kyubi would have been released and thus making the only thing the 4th did is killing himself so instead he sealed it into a baby, so when the 4th died the kyubi didnt go right back to destroying the village pointless if it had been don and he knew it