We know most of you are just as excited as we are about this and can't wait to try it out.
With 4.0, we've made some significant leaps in terms of modernization, and it's possible that you might need to do some preparation before you're ready to install it. Notably, our minimum PHP and MySQL versions have gone up. It's the first time we've needed you to do this in 6 years, and the versions we need you to have have been around for a long time, we're not requiring the latest versions.
In addition, 4.0 is UTF-8 only (if you don't know what that is, it's a way text can be stored in your database which you may or may not be using at the moment) and while the 4.0 upgrade process will ask you to convert your database if you're not already using it, this is a moderately time-consuming process, so if you convert your database now, it's one less thing to worry about on upgrade day.
To make this process as easy as possible, we have created a little script which you can upload to your server to test if you're ready.