release: bump version to 2.5.0-rc4

Bump version to 2.5.0-rc4.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/VERSION b/VERSION
index fe1944b..20419b3 100644
--- a/VERSION
+++ b/VERSION
@@ -2,4 +2,4 @@
 VERSION_MINOR = 5
 PATCHLEVEL = 0
 VERSION_TWEAK = 0
-EXTRAVERSION = rc3
+EXTRAVERSION = rc4