One-sentence summary; What is USB4
USB4 is the USB-C form factor for connecting; fully supports Thunderbolt 3 specification, all wrapped up using the USB 3.2 specification.
USB4 has four benefits over prior versions of USB.
Maximum Speed of 40Gbps. Using two-lane cables or a set of cables, devices may operate at up to 40 Gbps, the same speed as Thunderbolt 3. Keep in mind there is a big difference between Mb and MB. Mb is megabits, not Megabytes (MB). So for example 8Mb is about 1MB of data. As a reference, a typical MP3 audio file is about 3MB (megabytes). 5,000 MB/second is the theoretical maximum speed of USB4.
USB 4 supports DisplayPort 2.0. DisplayPort 2.0 cables feature 80Gbps bandwidth, making it possible to display ultra-high resolutions at previously impossible refresh rates. DisplayPort 2.0 can handle up to two 4K screens at 144Hz simultaneously, or an 8K display at up to 85Hz natively, with no form of image compression. This is true because USB4 uses all eight data lanes at once time.
Compatible with Thunderbolt 3 devices. USB4 is a protocol which supports all the specifications of Thunderbolt 3; however, Thunderbolt 3 is capable of 100Watts of bi-directional power delivery and not all manufacturers who support USB4 will not include the [full] power implementation of Thunderbolt 3.
Most efficient resource allocation scheme. USB4 devices use a process called “protocol tunneling” which optimizes the use of DisplayPort, PCIe and USB packets at the same time while allocating bandwidth to optimize efficiency. This scheme will create better performance across multiple devices with a collection of protocols.
USB4 will only operate through a USB-C type physical connector. USB4 peripherals will most likely not see older standard USB type A ports because the connection speeds and power delivery mechanisms will not be available. Although USB4 is 100% backward compatible with all other USB protocols, it doesn’t mean the older standard will get the improved benefits. If connecting, for example, a Type-A, 5 Gbps USB 3 port by using an adapter, the speed and power will drop to the lowest common denominator.
Some notable comments:
Device and host manufacturers will not [be required] to pay Intel royalties when implementing USB4 technology. This implies a better chance of mass adoption of USB4; however, there is a catch between manufacturing USB4 devices and making said products [fully] USB4 compliant. Specifically, the Thunderbolt compatibility specification may become a part of the product when developing and manufacturing a USB4 product which can use the USB4 logo. This issue means a consumer could buy a laptop with USB4 and find that it doesn’t work with a Thunderbolt 3 peripheral.
It is important to know Thunderbolt 3 and Thunderbolt 4 are logo certified programs from Intel which cost manufacturers time and money. So, while a USB4 powered computer could work with 40 Gbps devices or even those labeled as Thunderbolt, it may not be obvious because the product didn’t go through a certification process. Or the opposite of this would be a USB4 device does not support Thunderbolt because of the expense required to get the certificate.
USB4 has two speeds. As with Thunderbolt the paradox a USB4 product may not support the full 40Gbps specification. 40Gbps is the theoretical maximum speed, but many devices will use the lower 20Gbps standard because the manufacturing cost will be lower, thus creating a lower target price for consumers. If speed is the number one priority be sure and check the specifications of the USB4 product before purchase. At the time of this writing most USB4 products which support 40Gbps are cables and PCIe adapter cards.
Why the USB4 name?
An online article that summarized an interview with Brad Saunders the CEO of the USB Implementers Forum [USB.org or USB-IF] indicated the lack of space between “USB” and “4” is to focus away from USB version numbers and focus more on brand. This branding concept for USB is a good change, but afraid the history of all the USB versions of the past will continue to haunt them.