Does Assassin's Creed IV have a windowed or fake-fullscreen mode?
Does AC IV finally have a window or borderless-window/fullscreen-window mode?
Solution 1:
Unfortunately, AC4 does not support Windowed Mode or a Borderless Window.
You can try to use DXWND though, which is designed to force those type of games into windowed mode.
Solution 2:
Whoever made this is a legend: he made an application that can make AC4 run in windowed mode. You can use it, and it's better than DXWND:
Assassin's Creed IV Black Flag Window Mode Patcher.