Bump version to 2.1.0~rc4

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

configure.ac

@@ -2,7 +2,7 @@
 m4_define([gnt_version_major], [2])
 m4_define([gnt_version_minor], [1])
 m4_define([gnt_version_revision], [0])
-m4_define([gnt_version_suffix], [~rc3])
+m4_define([gnt_version_suffix], [~rc4])
 m4_define([gnt_version_full],
           m4_format([%d.%d.%d%s],
                     gnt_version_major, gnt_version_minor,