diff --git a/src/constants.hpp b/src/constants.hpp
index 8eeacd135..ea2e9a679 100644
--- a/src/constants.hpp
+++ b/src/constants.hpp
@@ -29,7 +29,7 @@
 
 const int major_version = 6;
 const int minor_version = 1;
-const int revision      = 3;
+const int revision      = 4;
 
 /// NIST value for the inverse fine structure (http://physics.nist.gov/cuu/Constants/index.html)
 const double speed_of_light = 137.035999139;