Re: Why Sarutobi-sama had to prevent the fourth's Coffin from opening...
The Sandaime Kazekage would have fit nicely there, however, making the pattern 1, 2, 3, ect.
The "but we didn't know anything about him yet" argument: We knew just as much as we did about the Sandaime Kazekage at the time as we did about the Shodai and Nidaime Hokages. The only difference: We knew what the Shodai and Nidaime Hokage's faces looked like. And the flashbacks of Sarutobi with Shodai and Nidaime? To intoduce them to us a little. Face it, before the fight, we knew just about as much about the Shodai and Nidaime Hokages as we knew about the Sandaime Kazekage: practically nothing. If the Sandaime Kazekage was first summoned here in his real body, instead of during the Sasori fight as a puppet, we would have learned about who he was there during the fight. Baki or someone would somehow noticed one of the Sandaime Kazekage's ninjutsus and say "That... couldn't be!" like one of the Anbu did when they noticed Shodai's jutsu.
The "Sarutobi was afraid of who was in the coffin" argument: Sandaime was already going to have to fight two other Kages and then Orochimaru, a very difficult thing to do. It didn't really matter who was in the coffin, he just didn't want to have to fight a total of four opponents. From the pattern, it's almost 99% chance of being true that the person in the last coffin was another Kage. If that summon had been succesful, Sarutobi would have had to fight a total of four Kages (Orochimaru=Otokage), which is nowhere near an easy thing to do.
And don't forget the Sandaime Kazekage's Iron Sand jutsus are just as dangerous as Shodai's mokuton jutsus and Nidaime's suiton jutsus. Fighting three people who use techniques such as those would be no where near easy, so he had to take down at least one coffin, to give himself a better chance of winning. Fighting four Kages would have just been too much for Sarutobi, and he knew he had to take one out, if he wanted a good chance of winning.
The "Sarutobi knew it was Yondaime" argument: And Sarutobi can't be wrong? He has some psychic powers that told him he was going to summon Yondaime? Summoning the Sandime Kazekage would have also completed and made a pattern. Sarutobi's hunch was that it was going to be Yondaime. However, it could have just as easily been the Sandaime Kazekage.
Character's can be wrong and make mistakes. I don't see what's so hard to grasp about this. After all, if this were right, I assume Sai must be right about Naruto making a mistake and only would have gotten in the way during the teamwork training exercise. And that Itachi must be right about how killing your best friend is the only way to get the MS. Face it, characters can make mistakes and be wrong and there is every chance that Sarutobi could have been wrong here.