The most reliable official sources for a digital download include:
: A confirmed platform for the upcoming Black Flag Resynced . Microsoft Store : Lists the Gold Edition for Windows.
: Provides the original game and will be a primary platform for the "Resynced" edition.
The original game is highly accessible for most modern hardware, while the "Resynced" version requires significantly more powerful components. System requirements for Assassin's Creed IV Black Flag