Update version for v4.0.0-rc4 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 859ede9..9c10dac 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-3.1.93
+3.1.94