This is not the original Didact here he has 6 fingers the original didact ( Ur-Didact) only had five. Bornstellar when he mutated thanks to the Didact had 6 fingers essentially Bornsteller gained the Didacts memories and ended up becoming the Didact. So bornstellar is apparently the one in Halo 4 well that is what a lot of fans say but I don't see how it is Bornstellar considering he never hated humanity
without giving the whole story away in the book cryptum Bornstellar and two humans Chakas and Riser released the didact from a cryptum that he was locked away in. They then go with the didact and bornstellar is then mutated. (Mutation was a Forerunner term refering to customized biological maturation to new forms or rates. The process was typically performed two to five times over the course of a lifetime) basically there is a forerunner that is there during the mutation that the mutation is actually based on so u end up looking like that forerunner usually it is done by the forerunners father or mother depending if they are female or male. But the mutation that is performed on bornstellar is called a Brevet mutation (Brevet mutation was a term used to refer to the undertaking of risky, ad hoc mutations that carried higher risk of complication) the Didact passes on his memories and part of his mind into Bornstellar, this allows the Didact to live on. I still haven't read the second book and the third is not out but im not to sure if the memories transferred to bornstellar actually makes him fully turn into the didact. But if it doesnt then that should mean that he cant be the one in Halo 4 because bornstellar liked the humans. I guess no one will truly know 100% until the third book is released.