In the anime, Kabuto reincarnated Asuma, Dan, Hizashi etc from Konoha to battle against the allied Shinobi forces. Earlier, Orochimaru reincarnated the 1st and 2nd Hokage to battle against the 3rd. Why didn't Kabuto reincarnate the four of them to fight against the alliance? Couldn't they be reincarnated again or wasn't that part of Tobi's plan?
Actually, Kabuto can't revive the 1st and the 2nd Hokage simply because Hiruzen Sarutobi (3rd) sealed them away using the Dead Demon Consuming Seal.
While the reason Orochimaru can revive the two is because he ripped the belly of the Demon (Shiki Fuujin) who consumed their souls in Naruto chapter 618, page 4.
The last time first and second hokage were reincarnated, they were sealed using the death reaper seal by the third hokage.
The 4th hokage was sealed along with the nine tailed fox using the same seal.
One of the pre-requisites of the reanimation jutsu is that the target shinobi cannot be sealed.
Since the 4 hokages were sealed, kabuto was unable to reincarnate either one.
However, later Orochimaru shows that the souls trapped in the death reaper's belly can be realesed, and thus reincarnated each of the hokages.