This is a major upgrade for the WordPress editor. There are many changes in 4.0:

New UI UI User interface and UI API API An API or Application Programming Interface is a software intermediary that allows programs to interact with each other and share data in limited, clearly defined ways. .

and UI . New theme.

Revamped events system/API.

Better code quality, readability and build process.

Lots of (inline) documentation.

And generally many improvements everywhere.

All default TinyMCE plugins have been upgraded. The WordPress implementation custom plugins were upgraded too. Looking in the pluginPlugin A plugin is a piece of software containing a group of functions that can be added to a WordPress website. They can extend functionality or add new features to your WordPress websites. WordPress plugins are written in the PHP programming language and integrate seamlessly with WordPress. These can be free in the WordPress.org Plugin Directory https://wordpress.org/plugins/ or can be cost-based plugin from a third-party repository, there are a lot of WordPress plugins that add a TinyMCE plugin. Because of all the API changes, most of these plugins would need an update too. If you are the author of such plugin, please test it in trunktrunk A directory in Subversion containing the latest development code in preparation for the next major release cycle. If you are running "trunk", then you are on the latest revision. now.

Generally there are three groups of TinyMCE plugins added by WordPress plugins:

Custom plugin created specifically for the WordPress plugin. If you’ve developed this kind of plugin, please see the 3.x to 4.0 migration guide and the 4.0 API documentation.

WordPress plugins that add third-party or default TinyMCE plugins would (of course) need to be updated to include the 4.0 version of the plugin. The PHP PHP The web scripting language in which WordPress is primarily architected. WordPress requires PHP 5.6.20 global $tinymce_version can be used to determine which plugin to load.

global $tinymce_version can be used to determine which plugin to load. Mini-plugins that only add a button to the toolbar. This works pretty much the same. It is advisable to update to use the ‘dashicons’ icon font instead of image icon.

TinyMCE 4.0 includes a ‘compat3x’ plugin that should prevent all fatal errors caused by old plugins and adds compatibility for most of the 3.x API methods. If there are any editor related JavascriptJavaScript JavaScript or JS is an object-oriented computer programming language commonly used to create interactive effects within web browsers. WordPress makes extensive use of JS for a better user experience. While PHP is executed on the server, JS executes within a user’s browser. https://www.javascript.com/. errors while running trunk, please open a tracTrac An open source project by Edgewall Software that serves as a bug tracker and project management tool for WordPress. ticketticket Created for both bug reports and feature development on the bug tracker. quoting the first error from the browser console.