WordPress 4.0 has just been released. Hurrah! However… if you have the capacity to update this yourself don’t do it just yet. The initial release is usually patched for security and bugs fairly soon after release, so we’ll be waiting a couple of days to see if this happens.
If not, we do still need to test sites fully before rolling it out. As it’s a major release, there’s a fair chance that some things have changed quite dramatically and updating without testing is a bad idea. Rest assured, however, that the last version (3.9.2) will continue to be supported for security updates for a while, and there’s absolutely nothing wrong with staying with this version for quite some time yet.
If you do update, always remember to take full file and database backups before doing so. Restoring sites if you don’t have these may be impossible, and we can’t be held responsible for updates carried out without testing and backups.