My general problem with QC is that the protocol is proprietary, this makes it difficult to find exact definitions (For some of the versions the chip datasheets shows everything, but they obvious figured that out and asked the chip manufactures to limit information in datasheets).
There speed definitions are just talk, many other fast charge protocols can match them.
Why QC4 is an advantage I do not yet know, they obvious try to stay within the USB-C framework, but I have not yet seen a specification on what they do and why it is better than just USB-C PD.