Update version to 18.3.0-rc4

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/VERSION b/VERSION
index bd71fb7..09c62b6 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-18.3.0-rc3
+18.3.0-rc4