bump sparse's version to 0.5.1-rc4

Signed-of-By: Christopher Li <sparse@chrisli.org>
diff --git a/Makefile b/Makefile
index 3a4e6b4..96e81c4 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-VERSION=0.5.1-rc3
+VERSION=0.5.1-rc4
 
 # Generating file version.h if current version has changed
 SPARSE_VERSION:=$(shell git describe 2>/dev/null || echo '$(VERSION)')