unable to run the install script.

You don’t need a full stop at the end of the address.

Are all your files and directories in the /blog/ subdirectory?

That is odd. It should work. The 404 message is only shown when you point the browser at a page that doesn’t exist. So that suggests that the file isn’t where you think it is.

As a test, you could try pointing at one of the other files, say http://example.com/blog/wp-admin/admin.php

If that gives you a 404 error too, I’d suggest uploading the files again to your host, in case something was skipped. Maybe even run through the detailed install.