Dexterity, Diazo, and a rock solid and very popular CMS still supported today
Plone 4 was based on the most frequently requested features and improvements from Plone's worldwide user and developer community. It came with a new, responsive theme, which in 2010 was something to behold. It delivered improvements in raw speed and scalability, including massively improved handling of large files and media. It also continued to advance its hallmark strengths of unmatched security and usability and had lots of under-the-hood changes to make Plone more future proof.
Plone 4 saw the introduction of Dexterity, a new and improved content type framework. Based on ideas hatched during the Plone 3 era at the 2008 Strategic Summit, the Dexterity type editor provides easily customizable, form-based content type creation in the browser. Site administrators can create and customize various content types for different needs with just a couple of clicks. The Dexterity framework introduced behaviors, which abstract discrete content type elements, be they fields or more general functionality, that can be applied to multiple content types. This is a powerful tool for developers, but in addition to that behaviors are accessible to administrators when they create new content types through the web.
In Plone 4 the Diazo theming engine was integrated and became the default way to theme Plone sites. Diazo allows a theme contained in a static HTML web page to be applied to a dynamic website, such as one powered by Plone. Diazo enables designers and developers to work in a more decoupled fashion, and removes the requirement for designers to understand anything about Plone or its page templates. In Plone 4.3, a theme editor was introduced, the product of much usability testing and collaboration with end-users. This tool allows Plone to be themed through-the-web, without touching the filesystem.
Plone 4.3 is still supported today and many 4.3 websites still exist.