Ok, I don't think the 4th's soul is sealed in eternal battle with kyuubi's. It is more likely that he just traded his life to seal kyuubi.

#1. We saw kyuubi inside naruto - but no 4th.
#2. The forbidden seals take different forms - we can't assume just because the hokage used the "same type" of seal - that it was the exact same thing. There are variations like waterbushin's and kagebushin's.

So vis-a-vis concordantly ergo - the 3rd coffin was yondaime : otherwise, it would be both yondaime's soul AND kyuubi's soul .... wouldn't that be scary? Two kyuubis? Maybe that was what sandaime was really concerned about... ?