Assassin's Creed 4: Blag Flag PC specs have been posted over on 3djuegos.
Mind you Ubisoft has not sent us a press release on this, so keep it as pure speculation for the time being - they could end up being wrong like the ones listed for is a bit larger than Watch Dogs.
Here's what the specs list states, per a translation from PCgamesN:
Operating System: Windows Vista SP SP1 or Windows 7 or Windows 8 (both 32/64bit versions)
Processor: Intel Core 2 Quad Q8400 2.6 GHz or AMD Athlon II X4 620 2.6 GHz
RAM: 2GB for Windows Vista, Windows 7, Windows 8
Video card: Nvidia GeForce GTX 260 or AMD Radeon HD 4870 (512MB VRAM with Shader Model 4.0 or higher)
DirectX: DirectX June 2010 Redistributable
Disk Space: 30 Gb
Sound: DirectX compatible sound card with latest drivers
Peripherals: Windows-compliant keyboard and mouse required, optional controller
Operating System: Windows Vista SP2 or Windows 7 SP1 or Windows 8 (both 32/64bit versions)
Processor: Intel Core i5 2400S 2.5 GHz or AMD Phenom II x4 940 3.0 GHz or better
RAM: 4GB or more
Video card: Nvidia GeForce GTX 470 or AMD Radeon HD 5850 (1024MB VRAM with Shader Model 5.0) or better
Supported video cards at the time of release: Nvidia GeForce GTX 260 or better, GT400, GT500, GT600, GT700 or AMD Radeon HD4870 series or better, HD5000, HD6000, HD7000 series*
Note: Latest GeForce drivers tested: 320.49 for all series. Latest Radeon drivers tested: 13.1 for Radeon HD4000, 13.4 for Radeon HD5000 and above.
*Laptop versions of the cards may work but are not officially supported.
Assassin’s Creed 4: Black Flag lands on PS3 and Xbox 360 October 29 in North America, and November 1 across Europe. The PC version is due on November 19.
The PS4, Wii U and Xbox One versions will be made available on November 22.