USB4 is based on the Thunderbolt 3 protocol specification and is backward compatible with USB 3.2 and USB 2.0. Comes in two versions - USB4 20 Gbit/s and USB4 40 Gbit / s.
USB4 Gen 2x2 supports 20 Gbit / s throughputs, is compatible with Thunderbolt 3 and is backward compatible with USB 3.2 and USB 2.0.
USB4 Gen 3x2 supports 40 Gbit / s throughputs, is compatible with Thunderbolt 3 and is backward compatible with USB 3.2 and USB 2.0.