This plugin allows you to automaticaly had watchers by category. A tab on the project menu will be available to configure the watchers
Clone or download the project to plugin folder (Be sure the folder is named ‘redmine_category_watchers’)
rake redmine:plugins:migrate NAME=redmine_category_watchers
Configure the permissions in the ‘Roles and Permissions’ page (under project)
P.S. I hope, it’s work on Redmine ver.4