I don't quite understand what your question is.
The xHCI does have a Supported Protocol Capability list and this list is used to indicate which port register set is to be used depending on the speed of the attached device. USB3 for Super-speed devices, USB2 for lower speeds.
If your specific xHCI card does not support any speeds above, for example, USB 3.0, it will only run at a max speed of USB 3.0. Just because the attached device can and may run at a higher speed, doesn't mean that the HC will run at that speed as well.
When you state that "it" returned a Speed Value of 4, do you mean a speed of 4.0? Probably not. A value of 4 indicates a speed of 3.0Gen1x1, or USB 3.0, Generation 1x1, which is a speed of 5 Gigabits/Sec.
Currently (circa Jan 2022), a value of 7 is the highest speed allowed, requiring a xHCI controller with an HC version of 1.2. This speed is 10 Gigabits/Sec with a name of "USB 3.2, Generation 2x2" or for short, "3.2Gen2x2".
Hope this helps,
Ben
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https://www.fysnet.net/the_universal_serial_bus.htm