Update version to 18.0.0-rc4

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/VERSION b/VERSION
index af01fd3..21a6c63 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-18.0.0-rc3
+18.0.0-rc4