After watching the last Terminal in halo CEA there is a lot of info on halo 4, it talks of the prometheans and if they where here they could handle the situation of humans destroying installation 04 but if they were there then he would have to be too, referring to the didact and he is there only the librarian can calm him, even if she is alive? the didacts mark appears in the UNSC infinities terminals when they are being sucked into requiem and the didacts symbol also appears around the text that says “an ancient evil” when he says i have been waiting a long time for this day reclaimer…
343 guilty spark also says he is worse than the flood and that the humans would prefer the flood over him. but 343 also says the ancient evil is the flood but 343i says the flood wont be in halo 4 unless they where lying or hiding the truth so they wouldnt be forced to give anything away. but the ancient evil to the forerunners would be the precursors. but it doesnt really explain who the evil is.
personally i think from seeing this terminal that the evil is the didact and his army of prometheans. just his logo appears a few places with the mentioning of evil, hearing his voice waiting for the cheif, it all ties together.