The G4 is a powerful RISC-based microprocessor, belonging to the PowerPC family of processors. It is used used in Apple Macintosh computers such as the G4 PowerBook (known colloquially as the "TiBook"), the 2002 2nd generation "Flat Panel" iMac and the desktop G4 Power Mac.

Most of the G4 design was done by Motorola in close cooperation with Apple. The name refers to the design being the "fourth generation" of PowerPC's from Motorola. IBM, the third member of the AIM (Apple Computer, IBM and Motorola) consortium, chose not to participate in the design of the G4 in part owing to microprocessor design philosophy disagreements concerning a Vector Processing Unit on the chip. Ultimately, the G4 architecture design contained a 128-bit vector processing unit called AltiVec (also known as "Velocity Engine" in Apple's marketing literature).

With the AltiVec unit, the G4 microprocessor can do four-way single precision floating point math, or 16-way byte math in a single cycle. Furthermore, the vector processing unit on the G4 is superscalar, and can do two vector operations at the same time. Compared to x86 microprocessors at the time, this feature offered a substantial performance boost -- if the application was coded to take advantage of the AltiVec unit.

Additionally, Motorola designed the G4 with enhanced support for symmetric multiprocessing. The G3 microprocessor line had some support for SMP, but computers using G3s in the SMP role took performance hits. By contrast, the G4 supports not only multi-processing, but also allows G4s used in SMP computers to pass data chip-to-chip in an extremely efficient manner.

Another big performance boost in the G4 microprocessor came from a 64-bit arithmetic logic unit, or ALU, derived in part from the 604 series ALU. The 603 and G3 series had 32-bit ALUs, which took two clock cycles to accomplish 64-bit floating point arithmetic.

The first version of the G4 microprocessor line was called the MPC 7400. It debuted in late summer of 1999 at speeds ranging from 350 to 500 MHz. The chip contained 10.5 million transistors and was manufactured using Motorola's 0.20 μm HiPerMOS6 process. The chip die measured 83 mm and featured copper interconnects.

Motorola's inability in 1999 to obtain yields of the G4 line at Apple's desired clock speed caused Apple to do an abrupt about-face on sales of its Power Macintosh G4 tower series of computers. The PowerMac series was downgraded abruptly from 400, 450, and 500 MHz processor speeds, to 350, 400, and 450 MHz. The incident caused a rift in the Apple-Motorola relationship, and reportedly caused Apple to ask IBM for assistance to get the production yields up on the Motorola G4 line.

The 1999 problems foreshadowed difficulities Motorola and Apple faced in competing with Wintel-x86 system clock speed increases, and the "Megahertz myth." It also perhaps ultimately caused Apple to release SMP versions of the Power Mac G4 series (with the ad campaign "Two brains are better than one") to make up for a perceived gap in performance between the Power Mac line, and competing x86-based systems running at higher microprocessor clock speeds.

As of early 2003, the fastest G4 processor shipping in Apple's Power Macintosh G4 line is the MPC 7455, running at 1.43 GHz. The MPC 7455-based Power Macintosh G4 is only available as a SMP version, with two processors.