PIP-0009: rev4 in header

PIP/PIP-0009.md

@@ -7,7 +7,7 @@
   Comments-URI: https://discord.gg/sJqcgtD  (channel #pip-0009)
   Status: Proposed
   Created: 2017-12-17
-  Updated: 2017-12-29 (rev2), 2018-01-01 (typos), 2018-02-06 (rev3)
+  Updated: 2017-12-29 (rev2), 2018-01-01 (typos), 2018-02-06 (rev3), 2018-08-17 (rev4)
 </pre>
 
 ## Summary