Mark Tails 4.0 as released.

debian/changelog

-tails (4.0) UNRELEASED; urgency=medium
+tails (4.0) unstable; urgency=medium
 
   * Major changes
     - Upgrade Tor Browser to 9.0-build2, based on Firefox ESR 68.2.
@@ -30,7 +30,7 @@ tails (4.0) UNRELEASED; urgency=medium
     - Make the "SSH is using the default SocksPort" scenario more robust
       (Closes: #17163)
 
- -- Tails developers <tails@boum.org>  Mon, 21 Oct 2019 08:46:36 +0000
+ -- Tails developers <tails@boum.org>  Mon, 21 Oct 2019 10:24:56 +0000
 
 tails (4.0~rc1) unstable; urgency=medium