While the PC requirements for Far Cry 4 weren't actually supposed to be revealed yet, Nvidia accidentally let the cat out of the bag earlier today. Fortunately, the specs were copied and pasted for the world to see.
Minimum System Requirements
Operating system: Windows 7 SP1, Windows 8/8.1 (64bit versions only)
Processor: Intel Core i5-750 @ 2.6 GHz or AMD Phenom II X4 955 @ 3.2 GHz
Memory: 4GB
Video card: NVIDIA GeForce GTX 460 or AMD Radeon HD5850 (1GB VRAM)
Direct X: Version 11
Hard Drive: 30 GB available space
Sound Card: DirectX 9.0c compatible sound card with latest drivers

Recommended System Requirements
Operating system: Windows 7 SP1, Windows 8/8.1 (64bit versions only)
Processor: Intel Core i5-2400S @ 2.5 GHz or AMD FX-8350 @ 4.0 GHz or better
Memory: 8GB
Video card: NVIDIA GeForce GTX 680 or AMD Radeon R9 280X or better (2GB VRAM)
Direct X: Version 11
Hard Drive: 30 GB available space
Sound Card: DirectX 9.0c compatible sound card with latest drivers

The system requirements were confirmed by the Far Cry Community Manager on the game's official forums.