I think he didn't want the 4th Hokage resurrected because he knew he wouldn't be able to beat him. Minato was better than the 1st and the 2nd (remember he was said to be the best of the best at that time). Whether it was with the other Hokage's or not he would've gotten served.
I agree the figure in the casket was someone new and someone who Madara feared because if it was someone he thought could be an ally, he would've probably had him resurrected.
I've seen some really great questions asked. I think the best question is to determine what is the relationship between the death reaper seal and the edo tensei.
I don't remember hearing explicitly that the third coffin Orochimaru tried to summon was the 4th hokage. Was that ever stated, or was it only guessed?
We know that the 4th hokage used the death reaper seal to seal the fox. But he was able to do it in a very complicated way: He was able to break the fox into two parts, and seal half the chakra in Naruto. He then also used a 4-trigrams seal around that to allow fox chakra to seep out. I don't think it was ever stated explicitly that the 4th sealed the second half within himself, but we can assume so. Maybe the 4th used a similarly advanced seal for himself, so we don't really know how it compares to the seal used by the 3rd.
so, If Orochimaru did summon the fourth, then edo tensei can summon victims of the reaper seal. So All hokages are currenly available to be summoned. The first hokage would be able to fight Madara evenly. The fourth hokage might still have half the fox in himself, half which Madara still needs to complete his plan.
Who else would Madara need, or who else ould he be afraid of? If Madara was the leaderof the second strongest clan, and if the only person who could defeat him was the first hokage with several bijuu and wood release, who would he be afraid of now. If this isn't the real Madara, would that matter? Maybe it would hurt his credibility in Akatsuki, but does that matter?
I don't think it's Orochmaru, since he isn't dead. He's trapped in slumber. At least, that's my opinion about it. Of course, if Kabuto is already becoming stronger than Orochimaru, what good would it do to summon somebody weaker than himself?
This is a huge mystery. I have no idea who it could be.