Thanks for getting in touch! It looks like the documentation page for enabling the guest account via the UI was not updated for 4.0. However, there is also a GitHub issue for a bug in 4.0 which is causing issues when enabling the Guest account via the UI. (Not entirely related to your problem as you are using the command line)
When using the Octopus.Server.exe command line, you do not need to specify --instance for the default instance. Could you try the command without the --instance flag and let me know the results? If the command is still erroring too fast for you to see, you could also try creating a .cmd file with the command in it, followed by a pause at the bottom of the command, to give you time to read/attach the error for us.
Let me know how you go, or if you have any further questions. :)