By default, the nvme is running at gen1 mode for compatibility, so the speed is limited. You can enabled gen2 mode by decompile and recompile the device tree:
fdtdump rockpi-4b-linux.dtb > /tmp/rockpi4.dts
Find pcie@f8000000 section
change from
max-link-speed = <0x00000001>;
to
max-link-speed = <0x00000002>;
Now recompile the dtb:
dtc -I dts -O dtb /tmp/rockpi4.dts -o /tmp/rockpi4.dtb
Replace the original rockpi-4b-linux.dtb(backup it first), reboot.