The error message tells you what you are doing wrong.
Either you are not running this command in the directory where you have
those files, or you do not have a file called Qubes-R4.0.4-rc4-x86_64.iso.asc - perhaps you gave it some other name
when you saved it?
I’m not supposed to do that? It’s what the instructions told me to do “If the filename of your ISO is Qubes-RX-x86_64.iso, then the name of the signature file for that ISO is Qubes-RX-x86_64.iso.asc, where X is a specific release of Qubes” Verifying signatures | Qubes OS .