How to convert rails3 routes into rails4

I have following rails3 routes that I want to convert into rails4.

map.with_options(:conditions => {:subdomain => AppConfig['admin_subdomain']}) do |subdom|
  subdom.root :controller => 'subscription_admin/subscriptions', :action => 'index'
  subdom.with_options(:namespace => 'subscription_admin/', :name_prefix => 'admin_', :path_prefix => nil) do |admin|
    ...
  end
end