Universal Serial Bus 4 (USB4), sometimes erroneously referred to as USB 4.0, is the most recent technical specification of the USB (Universal Serial Bus) data communication standard. The USB Implementers Forum originally announced USB4 in 2019.

USB4 enables multiple devices to dynamically share a single high-speed data link. USB4 defines bit rates of 20 Gbit/s, 40 Gbit/s and 80 Gbit/s. USB4 is only defined for USB-C connectors and its Type-C specification regulates the connector, cables and also power delivery features across all uses of USB-C cables, in part with the USB Power Delivery specification.

The USB4 standard mandates backwards compatibility to USB 3.x and dedicated backward compatibility with USB 2.0. The dynamic sharing of bandwidth of a USB4 connection is achieved by encapsulating multiple virtual connections ("tunnels") of other protocols, such as USB 3.x, DisplayPort and PCI Express.

USB4 is based on the Thunderbolt 3 protocol. However, it is different enough that backwards compatibility to Thunderbolt 3 is optional for many device types.

Prior to USB4, Thunderbolt provided a way to dynamically share bandwidth between multiple DP and PCIe connections over a single cable. Thunderbolt originally used the mDP connector and was only backward compatible to DP connections and did not support power transfer.

The introduction of the Type-C connector in 2014 provided a connector that could support USB data connectivity and power transfer as well as DP connections. It also allowed the static sharing of bandwidth between DP and USB connections over the same cable.

Thunderbolt 3 switched over to using the new Type-C connector and also added backwards compatibility for USB connections and power transfer features.

USB4 was announced in March 2019 by the USB Promoter Group. The version 1.0 of the USB4 specification, released 29 August 2019, is titled "Universal Serial Bus 4 (USB4™)". Several news reports before the release of that version sometime use the wrong terminology "USB 4.0" and "USB 4".