they dont use the bodies of the shodaime and nidaime...
they have live sacrifices the technique is called edo tensei or something like that...
so a grave of the fourth has nothing to do with it...

but im still sketchy on the fact that it was yondaime in that coffin, yeah it could be logic to say it was coz the shodaime and nidaime came out and the next logical choice would be yondaime as the only other dead hokage, but im just not sure you could summon his soul outta the death god, im not saying that it is impossible as we dont fully know the capacities of kuchiyose no jutsu, but i hope we do learn...
and if it is indeed possible it would be good to summon the hokages when naruto is older and have them fight him for orochimaru...and if indeed that yondaime is narutos father it would be pretty good, naruto will have to kill his father, and/or orochimaru..