Bump VERSION to v1.12.0-rc4

Signed-off-by: Tibor Vass <tibor@docker.com>
diff --git a/VERSION b/VERSION
index 3973f3b..c0cb6f1 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.12.0-rc3
+1.12.0-rc4