Release v4.0.0.
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 0a55f62..c080890 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -1,4 +1,4 @@
-v4.0.0 (UNRELEASED)
+v4.0.0 (2019-12-15)
 ===================
 
 - Require Python 3.5 or later.