The changelog notes security fixes to post by email, an XSS vulnerability in the Likes module, and the REST API/Contact form.
The update also improved features like developing on Kinsta, Jetpack for multisite, and connection process. There are also a variety of bugs that are taken care of improved compatibility.
4.0.4 also offers better access to customer support. There is now a contact form in admin that goes directly to the customer service team, making responses much faster.
If you haven’t already, make sure you update immediately from your dashboard.