Difficulty updating WordPress from 3.9.1 to 4.0
You need to check your file/folder permissions. Your webhost may have them set too strictly for WordPress to write its own temp folders and extract its own archive in /wp-content/upgrade/. See Changing File Permissions « WordPress Codex: Typically, all files should be owned by your user (ftp) account on your web server, and should be writable by … Read more