That’s a fitting single word descriptor for 4 point Oh.
Some of the highlights:
- a scrollable grid of images and document thumbnails in the Media section
- editing area that fits your viewable screen area and scrolls with you
- easier visual / search / installation features in the plugin directory
- lots of performance enhancements and bug fixes
The overall look & feel in the Dashboard is a bit more streamlined and clean-looking as well.
As with most major steps in software release, there are a few discrepancies between WordPress and the thousands of plugins & themes developers have released to work with WordPress, but for the most part, this was a rather painless update/upgrade, all things considered.