@Anonymous User
Have a quick read of the help guide article linked below, then if you are still getting no joy in adding your fourth devices you would need to jump over to live chat and explain to them what's happening where they should be able to assist you further.
To contact live chat open the link page below, click on the blue "Get in touch" button and click on the most relevant topic presented on screen until you reach the live chat option button.
https://help.nowtv.com/article/managing-my-devices
