Can't upgrade to WordPress 4.0

I’ve tried multiple times to upgrade to my current version of WordPress from 3.9.1 to 4.0, but every time I do, I get this message:

“Downloading update from https://downloads.wordpress.org/release/wordpress-4.0-no-content.zip…
Download failed.: Failed to write request to temporary file.
Installation Failed”

I’ve attempted to do the one click upgrade and transfer files via FTP, but nothing seems to work.

Any suggestions? Thanks!