In Assassin’s Creed 4: Black Flag we are no longer playing as Desmond in the real world using a magical machine to transport his consciousness back in time. Instead we are an employee of Abstergo Entertainment using a magical machine to transport our consciousness back in time. The difference being all the modern day stuff will be optional.
However, for those inclined to do so game director Ashraf Ismail says the modern day setting will provide three to five hours of content. his includes “finding everything, hacking every computer, accessing every room.” The only mandatory part of the setting is the ending 20 minutes or so. Presumably the modern day setting is to provide back story and further details on the overall arc of the Assassins/Templar war and maybe some resolution as to what happened after the end of ACIII.
Assassin’s Creed 4: Back Flag will come to the PS3 and current gen platforms on October 29th in North America and November 1st in Europe, while on the PS4 it will launch on November 22nd.