Bump version to 1.28.4

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/Makefile b/Makefile
index 7588166..9a41174 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 VERSION = 1
 PATCHLEVEL = 28
-SUBLEVEL = 3
+SUBLEVEL = 4
 EXTRAVERSION =
 NAME = Unnamed