Very close!
More details at Tip: Escaping curly brackets for substitutions in ActionScript
You need two open-curlies {{ at the beginning to avoid starting a Relevance Substitution.
You only need one close-curlie } at the end, because you are not inside a relevance substitution.
If this were inside a relevance substitution and needed to include a literal “}” as part of a string, you would have needed to escape that as “}}” to avoid ending the substitution early.
Try
copy "__Download\uninst_setup.iss" "C:\Program Files\InstallShield Installation Information{{4D9CA1B8-5FF5-47A7-8BDF-C37D1F9F55A5}"
I will also say that what you’re trying looks unusual…not sure why you’d need to manually copy files into InstallShield’s cache area, but be aware the copy will also fail if the target file already exists, so you may need to use the ‘delete’ command first before trying the ‘copy’ command. ‘copy’ will not automatically overwrite an existing file.