The *official* Quake 4 system requirements have finally been announced over at the official website, which you can also alternatively find here:

Microsoft Windows 2000/XP OS

Intel Pentium 4 2.0 GHz or AMD Athlon XP 2000+ CPU

512MB of RAM

8x speed CD-ROM optical disc drive and latest drivers

2.8GB of uncompressed, free hard disk drive space available (plus 400MB for a Windows swap file)

100% DirectX 9.0c compatible 16-bit sound card with latest drivers

100% Windows 2000/XP compatible mouse, keyboard, and latest drivers

3D hardware accelerator card required

100% DirectX 9.0c compatible 64MB hardware accelerated video card and the latest drivers

-ATi Radeon 9500/9700/9800 series
-ATi Radeon X300/X550/X600/X700/X800/X850/X1300/X1600/X1800 series
-nVidia GeForce 3/Ti/4/Ti series
-nVidia GeForce FX/6/7 series

The official Quake 4 Mac OS X port system requirements and recommendations have been just revealed over at Aspyr, and here they are for those Mac OS X users:

System Requirements:

Operating System: Mac OS X 10.3.9 or later

CPU Processor: PowerPC G4/G5 or Intel chipset

CPU Speed: 1.67 GHz or faster

Memory: 512 MB or higher

Hard Disk Space: 3.0 GB free disk space

Video Card (ATI): Radeon 9600 or better

Video Card (NVidia): GeForce 6600 or better

Video Memory (VRam): 64 MB or higher

Multiplayer: Internet (TCP/IP) and LAN (TCP/IP) play supported. Internet play requires broadband connection.

Media Required: DVD Drive

*Supported Video cards: NVIDIA GeForce 6600, 6800, 7800 ATI Radeon 9600, 9650, 9700, 9800, X600, X800, X850, X1600 Recommended System Requirements: Quake 4 runs best on a Power Mac G5 or Intel 2.0 GHz or faster

Operating System: Mac OS X 10.4.4 or later

CPU Processor: PowerPC G5 or Intel chipset

CPU Speed: 2.0 GHz or faster

Video RAM: 128 MB

As so little Quake IV information is available we will base it from what we know, considering Quake IV uses the DOOM 3 engine, we shall list the minimum specifications for the engine.  This will most likely be a lot lower then what Quake IV will require.

Minimum:
P4 1.5GHz CPU (or equivalent).
384MB of RAM.
64MB graphics card.

Lowest supported GPU is a GeForce 4 MX (worse than GeForce 3).
Supported cards:
GF 4 MX.
GF 3.
GF 4.
GF FX (and higher).
Radeon 8500s, 9000s and higher.