Not sure if this issue is strictly on the PS4 but upon reaching Sequence 12 Memory 2 , my game would crash right as the level was about to load with the error code CE-34878-0. The only way to get my game to load and actually be playable was to completely disable my Internet connection in the PS4’s settings.
The path to the setting is:
On the PS4’s home screen select Settings – Network – Uncheck the “Connect to Internet” Box.
Until Ubisoft releases a patch this may be the only workaround which isn’t really that bad as the game will continue to save locally and you get to continue and eventually finish the single player story mode. The down side to using this fix, you won’t be able to play multiplayer and the companion app will no longer function. A few other things like using Boosts and accessing Initiates, the store and certain features on the map will no longer function either.
This is a temporary solution to this bug as the game is rendered useless until I disabled my internet connection so whatever ACUnity is tying to access on the Ubisoft servers is either not there or sending back a bad reply and causing the game to ultimately crash. This bug is absolutely an Assassins Creed Unity bug as I could play any other title just fine with the internet connection re-enabled.
I Hope this helps to at least get things going again for you if your game was crashing and basically useless as mine was. Leave any comments or questions and I will answer and help you as best as I can regarding this issue.