Fix missing token in README.md

Author: NT7S

README.md

@@ -156,7 +156,7 @@ Here are the defines, structs, and enumerations you will find handy to use with
 
 Defines:
 
-    JT65_SYMBOL_COUNT, JT9_SYMBOL_COUNT, WSPR_SYMBOL_COUNT
+    JT65_SYMBOL_COUNT, JT9_SYMBOL_COUNT, JT4_SYMBOL_COUNT, WSPR_SYMBOL_COUNT
 
 Acknowledgements
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