Topping the new feature list is the introduction of whole new plugin-free charting package (which Sench uses as an example of the sandboxing mentioned earlier by providing a chart written in ExtJS4 running fine within an ExtJS3 desktop) built on web technologies (SVg and VML). Also, the Grid, which i have used extensively in ExtJS3 is getting a makeover and has been modularized and is now extensible. Some of the extensions used in ExtJS3 and previous versions to enable for example column locking or cell editing are now obsolete as those features are available out of the box. Even better yet, the markup used to render the grid, which turned out to be quite a challenge for me as it was not 508 compatible, is now generated on demand which makes the need for pagination unecessary. TreeGrid has been integrated within the core code and supports all of the normal Grid functionality.
The Data package has been revamped and supports HTML5 localStorage and introduces the new Model class which allows you to accurately model your data layer. I’ve somehow played with its Touch counterpart, and will probably expand on it later, but it consists mainly of Fields, which represent the data fields, a Proxy, which is responsible for loading and saving data from a source, Associations which defines how Models relate to each other, and Validations for validating fields before a save. This very much reminds me of the CakePHP MVC architecture, which itself is based on the RoR model.
I would mention in passing improved support for theming, a new FocusManager class which makes it easier to enable keyboard navigation in your application, which should really help with with accessibility and 508 compliance.
Overall, I think that Sencha is maintaining a good direction with their product and as soon as I find a good project whose requirements fit the features offered by the framework, it will be on top of my list of possibilities. In the mean time as I am working on some mobile development with Sencha Touch, it will allow be to familiarize myself better with all the new features.
Check out the press release for yourself: http://www.sencha.com/blog/ext-js-4-final/