AGP 4x refers to the initial variation of the Accelerated Graphics Port (AGP) interface. It was specified by Intel (specification v2.0) to improve the performance over AGX 1x and AGP 2x. It operates on a 32-bit channel at 66 MHz, which is quad pumped to 266 MHz, which provides a maximum transfer rate of 1066MB/s, compared to AGP 1x maximum rate of 266MB/s and AGP 2x rate of 533MB/s.
