In V3, basic content management was just pages and libraries. Other functions (for example, news articles, events, forms) all required special plug-ins that each had their own user interfaces and rules.
In V4, content management has expanded to include all of those special functions. So it is not just pages and libraries, but also calendars, blogs, member directories, surveys, galleries, courses, email blasts, and more. That means that all of those disparate plug-ins now share a single common basis, which means that updates, translations, sorting, e-commerce, access control, searching, workflow, and other CMS functions work the same across all of them. The user interfaces to these tools will be much more consistent, and learning how to use new tools should be much easier.
Since everything is now content, that also means that everything now has a specific place where it lives in your site map. That means that:
When configuring content, use the "Access" field (in the Access Controls pane) to control who is allowed to view it. Anyone of a lower access level will be given a login form instead.
You can set the access level of sub-elements of a page, so that the overall page is visible to one level, but a particular sidebar or box is visible to a higher level only.
The visibility of a page (or any other content) has more options in V4:
V4 is smarter about knowing whether content should be static (ie. fast & secure) or dynamic (flexible and personalized). Generally speaking, there is no need to specify which mode should be used for any piece of content. The content itself can figure it out. Just leave the "Publish Rule" setting blank unless you know you want something different.
V4 has a new concept - "interval publishing". This is halfway between static and dynamic - technically it is static content (ie. fast & secure) but it republishes on regular intervals so that it automatically refreshes with updates without you needing to remember what has to be published. You can select hourly, daily, and weekly intervals. (Or not – as noted above, the system will choose something appropriate for you.)
V4 is quite flexible in how content gets formatted (ie. the specific mark-up tags that are used to present the content to the viewer). Useful default formats are in place, but they can be altered or customized, if necessary.
For example, if you don't like how a specific type of content (for example, events) is marked-up/laid out, you can change it system-wide, for a specific website, or even for a specific calendar.