Update version to 17.3.0-rc4

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/VERSION b/VERSION
index 0c573cb..fac25a5 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-17.3.0-rc3
+17.3.0-rc4