If you're sharing sessions between a rails 3.x and 4.x application, this code can be dropped into the rails 4 app to work around the flash message incompatibility.

session_store.rb

# We override some fo the Flash code here to ensure our flash messages from this app are written to a different 
# session key than the default. This prevents conflicts with our older rails applications
require "rails"

ActionDispatch::Request.class_eval do 
  def flash
    @env["#{Flash::KEY}_four"] ||= Flash::FlashHash.from_session_value(session['flash_4'])
  end
end

ActionDispatch::Flash.class_eval do
  def call(env)
    @app.call(env)
  ensure
    @key = self.class.const_get(:KEY)
    session    = ActionDispatch::Request::Session.find(env) || {}
    flash_hash = env[@key]

    if flash_hash && (flash_hash.present? || session.key?('flash_4'))
      session['flash_4'] = flash_hash.to_session_value
      env[@key] = flash_hash.dup
    end

    if (!session.respond_to?(:loaded?) || session.loaded?) && # (reset_session uses {}, which doesn't implement #loaded?)
      session.key?('flash_4') && session['flash_4'].nil?
      session.delete('flash_4')
    end
  end
end

What does the KEY stand for in line 7 and 15 ?

It looks like line 17 should be "#{env[@key]}_four" since that's what the key is on line 7

--- a/config/initializers/session_store.rb
+++ b/config/initializers/session_store.rb
@@ -18,7 +18,7 @@ ActionDispatch::Flash.class_eval do
   def call(env)
     @app.call(env)
   ensure
-    @key = self.class.const_get(:KEY)
+    @key = "#{self.class.const_get(:KEY)}_four"
     session    = ActionDispatch::Request::Session.find(env) || {}
     flash_hash = env[@key]

(just FYI)