Epi 72 showed us where the 4th is, his spirit trapped with in that spirit thing from the sealing jutsu. You c him clearly...and then they show a flash of his face when the 3rd is speaking about there is always someone to inherit the will of the Hokage and he says ''isnt that right yondaime?'' (or something like that i h translate badly)

its pretty cool that all the hokages are in the same place at the same time.