Today we get Android 11 Developer Preview Four... Previously, Google said there would only be three Android 11 Developer Preview releases but Google is making a few changes to the Android 11 release schedule, and that includes this unexpected additional developer preview. The next beta release has been moved to June 3rd, where it will be dropped as part of a new Beta Launch Show online event details for which Google published earlier today. All of the following betas on the original schedule have also been pushed back a month.
Google says that this new timeline is the result of challenges posed to developers as a result of current events. The extra month will give them more time before Android 11's changes reach the "platform stability" milestone, and an extra month for developers to get Android 11 apps ready for testing (developers should note: SDK and APIs for Android 11 are still set to be finalized in June).