gazebo: add gazebo4 tag

library/gazebo

@@ -1,5 +1,10 @@
 # maintainer: Nate Koenig <[email protected]> (@nkoenig)
 
+gzserver4:      git://github.com/osrf/docker_images@d2eb18bf4ecfdf9116ed0893da4aa25a9ea38ddb gazebo/gazebo4/gzserver4
+libgazebo4:     git://github.com/osrf/docker_images@d2eb18bf4ecfdf9116ed0893da4aa25a9ea38ddb gazebo/gazebo4/libgazebo4
+# Docker EOL: 2017-04-30
+# LTS
+
 gzserver5:      git://github.com/osrf/docker_images@c7e0ac67d507b13e8b676a3d648051997a871f6b gazebo/gazebo5/gzserver5
 libgazebo5:     git://github.com/osrf/docker_images@c7e0ac67d507b13e8b676a3d648051997a871f6b gazebo/gazebo5/libgazebo5
 # Docker EOL: 2019-04-30