Just Because It Is One, Doesn’t Mean It’s The Other
USB has become a rather confusing interface, with more flavours of USB than a Baskin Robbins has of ice cream and while most are compatible with each other you need to pay attention to the versions available on your motherboard to ensure you are getting the full transfer speeds. To make it even more complicated, Thunderbolt 3 and USB-C 3.2 Gen2 are often described as the same, even though the first can hit 40Gbps with a proper cord while the second peaks at 20Gbps.
USB4 is not going to change that apparently, as a number of the controllers which have been released are compatible with one standard but not the other. Intel’s Maple Ridge Thunderbolt 4 controller, JHL8340, is not marketed as being USB4 compliant even though they are compatible and some of their SoC implementations seem to be actually certified. The ASMedia ASM4242 is expected to be one of the more common USB 4 controllers, yet it does not show on the certified list yet where as AMD’s mobile Ryzen 6000-series CPUs are certified.
The biggest difference between Thunderbolt 4 and USB4 is PCIe Tunnelling. In order to be Thunderbolt 4 certified the controller must support PCIe tunnelling, the ability to dynamically split available bandwidth to attached devices based on their needs. This will make chains of Thunderbolt 4 devices much more useful, as the 40Gbps will be split properly between the attached devices. PCIe Tunnelling is optional for USB4, so some controllers will support it but not all of them.
The bandwidth on the other hand is nowhere near as simple. You will soon see USB4 20 Gbps and USB4 40 Gbps devices both of which are backwards compatible with USB 3.2, 10 Gbps making the plug you chose even more important than it has been. Thunderbolt 4 will always be capable of 40 Gbps, with the proper cable of course. TB 4 will also be able to provide up to 15W power compared with USB4’s 7.5W. There are a couple of other benefits to TB 4 which you can see in the image.
The other new feature you can expect to see from both interfaces is the death of DP Alt Mode. USB4 will now be able to send PCI-Express and DisplayPort signals in addition to USB data natively. Just like Thunderbolt, USB4 will send encapsulated DP 11.4 data within a USB data stream, and it is only extracted once it reaches its destination DP 1.4 device. The data will also be prioritized over other traffic on that interface.
There is more to learn about how these major features work as well as many other new minor feature in this in depth look over at TechPowerUp.
USB4 is the next evolution of the USB standard, but it's quite different from its predecessors as it brings with it support for DisplayPort connectivity, as well as optional PCI-Express and Thunderbolt 3 support. It tries to fix some technical issues, but adds a few of its own, but at least the labeling is clear.