Also called "Ultra 320" or "Fast-160". Again doubles the data throughput to 320 MBytes/sec. Defined in the SPI-4 document of the SCSI-3 specifications, Ultra 320 requires LVD signaling. To reduce overhead and increase data throughput Ultra 320 will include QAS and Information Units (packetizing). Maximum allowable Ultra4 cable length is 12 meters (39.4 feet) with more than two active SCSI IDs and up to 25 meters (82 feet) point to point. Ultra320 is backward compatible through the single-ended interface using Ultra or slower speeds and cable lengths. As with Ultra3 SCSI, single-ended is not defined at Ultra4 throughputs and HVD is obsolete.

This definition is in context to SCSI. See more contextual defintions for Ultra4 SCSI.