diff --git a/src/digraph.c b/src/digraph.c
index fb45c15..0a4a4a3 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -853,6 +853,7 @@
 	{'1', '\'', 0x2032},
 	{'2', '\'', 0x2033},
 	{'3', '\'', 0x2034},
+	{'4', '\'', 0x2057},
 	{'1', '"', 0x2035},
 	{'2', '"', 0x2036},
 	{'3', '"', 0x2037},