Core version bump to 4.0.0
diff --git a/build.yaml b/build.yaml
index 6e590f4..5b701a8 100644
--- a/build.yaml
+++ b/build.yaml
@@ -12,7 +12,7 @@
   '#08': Use "-preN" suffixes to identify pre-release versions
   '#09': Per-language overrides are possible with (eg) ruby_version tag here
   '#10': See the expand_version.py for all the quirks here
-  core_version: 3.0.0-dev
+  core_version: 4.0.0-dev
   g_stands_for: gentle
   version: 1.3.0-dev
 filegroups: