cap.rb

role :app, "app4"
role :web, "proxy"

after "deploy:assets:precompile", "assets:copy_manifest_file"

namespace :assets do
  task :copy_manifest_file, :roles => :web, :except => { :no_release => true } do
    app_servers = find_servers :roles => :app

    app_servers.each do |app_server| do
      run `scp #{release_path}/public/manifests.yml #{user}@#{app_server}:#{release_path}/public/`
    end
  end 
end

What you know about asset compilation?