You didn't state what processor you have currently.
Yes, PCIe gen4 is supported.
I don't quite understand your last question. The C8H uses the X570 chipset. "Newer" boards use B550 chipset.
You can compare both chipsets online, but in a few words the X570 (and thus the C8H motherboard) is the most feature rich chipset to date.
Personally I would not change the C8H for a board with B550 chipset.
Cheers.