Prepare for v0.4.0. (#26)

diff --git a/README.md b/README.md
index 2306565..96bb9f7 100644
--- a/README.md
+++ b/README.md
@@ -122,7 +122,7 @@
 
 ```yaml
 - repo: https://github.com/google/keep-sorted
-  rev: v0.3.1
+  rev: v0.4.0
   hooks:
     - id: keep-sorted
 ```