You guys need this because the game just launched and people are already asking about optimal PC builds! Forget wrap/WidescreenFix for now since it's a native Remaster with its own display scaler, so set your resolution to Native at 4K if you can — don't force Widescreen. They also added Ray Tracing this time around; toggle them on and off in the separate RT menu rather than a global master switch because that lets you keep high textures while selectively turning down ray-traced shadows, which I highly recommend for mid-range cards. The graphics tiers are 4K Ultra (Ray Tracing), High, Medium, Low, Off — each drops specific features like texture quality and shadow resolution; the only thing genuinely not playable at 1080p is 'Off' because everything else still renders beautifully. If you have an older GPU, stay on High with Ray Tracing disabled to keep your frame rate stable without losing much visual fidelity since they remade almost every asset anyway!

Source: https://www.ign.com/articles/assassins-creed-black-flag-resynced-pc-settings-guide