In Naruto, we have seen that Orochimaru summons the coffins of both, the 1st and 2nd Hokage, and uses them to fight against the 3rd Hokage. Also in Shippuden Season 2, he tells that he experimented with the 1st Hokage's body to make Yamato. Why didn't the coffins of both the Hokage have any seals on them, so that no one could humiliate their bodies?
How you perceive it is incorrect.
The Edo Tensei doesn't summon their coffins or their dead bodies. Edo Tensei uses the body of a live person. Then it binds the soul of the person you want to ressurect to the body of the host. Through this technique, they get the image of their own body and can use their jutsu.
The reason the Fourth couldn't be summoned was because his soul was sealed through a sealing technique he used to seal the kyuubi.