Comment USB4 in a Nutshell (Score 5, Informative) 78
Reading through the comments, there is a bunch of misunderstandings about USB4. Given the poor quality of the press coverage on it, I am not surprised. So let me clear things up.
#1 - USB4 is almost identical to Thunderbolt 3
USB4 is actually an "alternate mode" that gets negotiated via USB PD. The protocol and electrical signaling for USB4 is completely different and totally incompatible with USB3. USB4's protocol and signaling is an exact copy of Thunderbolt 3. USB4/Thunderbolt 3 follows the OSI model; it has a generic data transport physical layer and flow control protocol; on top of that you can tunnel many different protocols. Just like how you can tunnel TCP/IP, IPX, AppleTalk, NetBIOS, etc. over Ethernet; You can tunnel PCIe, DisplayPort, and Ethernet data packets over the USB4/Thunderbolt 3 physical layer. USB4 adds one very important thing that the original Thunderbolt 3 lacked... in addition to PCIe/DIsplayPort/Ethernet USB4 can also tunnel USB data packets. Other than that, the only difference between USB4 and Thunderbolt 3 is that Thunderbolt 3 has a different Alternate Mode ID number. Backward compatibility with USB3 devices is achieved by not activating the USB4 alt mode. USB1/2 has dedicated pins on the USB-C connector, that gives backward compatibility all the way back to 1994.
#2 - USB-C is a fundamental requirement
Since USB4 is an alternate mode, and alternate modes do not exist for the older connectors (USB-A, Micro-USB, etc.) it is technically impossible to implement USB4 on top of USB-A.
#3 - USB4 hubs are going to be crazy complex (aka spendy for the first couple years)
The USB4 specification requires USB4 hubs to be backwards compatible with both USB3 AND Thunderbolt 3. Moreover, when running in USB4/Thunderbolt 3 mode, all hubs are required to support PCIe and USB traffic. That means that every hub needs to implement not only a USB hub, but a PCIe switch as well. The Thunderbolt 3 compatibility requirement also means that every hub needs to have an integrated XHCI controller, since Thunderbolt 3 can't tunnel USB packets natively USB traffic has to be converted to PCIe traffic to go over the Thunderbolt bus.
USB4 devices are not required to support Thunderbolt 3 backward compatibility however. This is going to result in some pretty weird behavior. If you plug a USB4 only device directly into a Thunderbolt 3 USB-C port on a laptop... it won't work. But... if you plug a USB4 hub into that Thunderbolt 3 port, and then plug the USB3 only device into the hub... it will work!!! This will be mitigated somewhat by the fact that most USB4 devices will probably support backwards compatibility with USB 2/3, but still be prepared for lots of confusion from less technical users.
#4 - PCIe Tunneling is a standard feature, and its secure
Since USB4 is the same as Thunderbolt 3, PCIe tunneling comes standard. And no, this a NOT a security issue. Microsoft implemented support for IOMMU assisted IO Virtualization in WIndows 10 1803/RS4, so the "Thunderstrike" security issue has been fixed. Apple did a similar fix on macOS ~8 years ago. No idea what Google's plans are for implementing IOMMU support in Android/Chrome OS, so there is a chance that PCIe tunneling will be disabled in firmware on stuff running on Google OSes... hopefully not as that would just be one more compatibility headache for everyone.