OK, it’s worth reminding anyone who doesn’t keep up with such matters that WordPress version numbers are purely decimal. 4.0 is just the next number after 3.9 rather than a Huge Major New Version
But having said that, this new one does contain some interesting enhancements to the editor, which is where most WordPress users will spend most of their time. It’s long been possible to embed such things as videos, images and even tweets into posts, but up to now, all you’ll have seen in the editor is a URL. Now, these things are displayed so you can see what they’ll look like before saving a draft and hitting the preview button. Or publishing, if you’re not that cautious or not that bothered (or both). Here’s one I did earlier:
WordPress 4.0 – site updated without any apparent borkage. This update seemed to run pretty quickly, too. Good stuff. #wordpress
— Les Bessant (@lesb) September 4, 2014
Interesting. First, a box appeared, then the text, then the nicely formatted version. Nice. I like.
The next thing is a rather nice improvement – rather than being a fixed scrolling window, the editing box expands to accommodate larger posts, gradually expanding down the screen, letting you see more of your words at once, which I rather like.
There is also a change to the Media Library, which now displays images in a grid rather than a list. I rarely look in there, so that’s not important to me, but might be for other users.
While you should, of course, make sure you have a backup before upgrading anything as complicated as WordPress, you should find the upgrade process to be quick and painless.
If your server is configured to allow it, you should be seeing the upgrade prompt in your dashboard, otherwise a visit to the WordPress site, where you can read more about the new version, is recommended:
 About five minutes earlier, that is
 Unless you’ve got some unusual combinations of plugins, theme and server config, in which case take care…