use your edit button in the bottom right hand corner of your post instead of double posting


and the fourth probably felt no need or did not know he would be sealed inside the demon gods body

most likely the fourth used a shiki fuuijin to remove kyuubis soul a second seal to place kyuubi into naruto and seal it instead of letting the death god eat it

and maybe used another seal to place the fourths soul inside of naruto but i doubt it cause i doubt the death god would go for that i bet your locked into having you soul taken as soon as you use the jutsu