1) The blue "Spirit" Trapped by the God of Death is Sarutobi's Soul...
2) The Spirit/Soul of the 1st and 2nd are held within those two "containers" or sacrifices by the Edo Tensi, in order for them to be dealt with, Sarutobi needed to remove their souls from the Jutsu, so he sealed them within his two clones. If you take note in Episode 73, also in the manga, the God of Death slices the two clones and the souls of the 1st and 2nd and eats them...thus, taking the clone energy/soul from Sarutobi, and the clones.
3) We have no idea exactly what type of technique was used on Kyubi, however, it must be similar to the one Sarutobi used, I explain it like this:
Its the Jutsu of the 4th, however, they are two different level techniques used, One seals the soul into another host (There could be more to this one...), the other seals the soul in your body, then the God of Death takes both.
If you refer back to the part of the Manga where Jiraiya was teaching Naruto the Resengan, he says there are six stages, you need to get to three to master this technique...Perhaps the 5th stage is where you seal the soul into yourself, and the sixth is sealing it into another host...could be totally wrong, its only speculation until we are shown what happenes...
4) So far, based on both the Anime and the Manga, there is no evidence to suggest Naruto is a reincarnation of the 4th, or else he would be referred to that way by everyone...Besides, the 4th wanted Naruto to be seen as a hero, so there are two distinct people we are talking about here.
- Reason -