Comment by twerquie

Put a reverse proxy server such as Nginx in front of your existing Rails 2.3 servers as well as a new Rails 4 app server. Slowly, route by route, port controllers over to the Rails 4 application and map traffic there using Nginx rules. This method is completely safe: If you notice discrepancies, you can point traffic back to your Rails 2.3 environment while you improve the behaviour of the Rails 4 replacement. Lather, rinse, repeat until it's all ported over.