I had the same problem many have been posting about the game crashing on start.
I'm not too Windows sauve, so I'll do my best to explain how I fixed it.
Under the Event Viewer, the error event had "Faulting application path: C:\Program Files (x86)\Steam\steamapps\common\Assassin's Creed IV Black Flag\AC4BFSP.exe
Faulting module path: unknown"
Which was pretty useless information, but I clicked on the following event and got the information
StackHash_0a9e, was the only thing that seemed useful at all so I googled it and found the following forum posthttps://forums.geforce.com/default/topic/549500/solution-for-stackhash_0a9e-errors-and-possibly-others-/
The instructions were to
"Go to Control Panel -> Programs and Features.
On the left-hand side, click on 'View Installed Updates'.
Under the 'Microsoft Windows' section, search for and uninstall KB2670838
Restart computer when prompted."
After doing this the game loads fine.
This may have also fixed my COD: Ghosts from not crashing, but I'm not sure if a patch was released while I was away from my rig for thanksgiving.
I hope this helps people.
Also if it does, please pay it forward and post the solution in other threads you have contributed to while troubleshooting the problem.