As of the time of posting this, WordPress.org still is showing 3.9.2, but 4.0 is slated to be released today. I hadn’t even known today was the day till I saw the article below linked on Twitter.
Browsing through the article below, it doesn’t look like 4.0 is a major upgrade from a feature set. I’m sure there’s a ton going on below the hood though.
With any upgrade, mission critical upgrades should wait to make sure bugs are squashed and that all your plugins are compatible.