Large database 4GL applications

I was recently contracted to work with a company that has developed a very large, (one million lines), 4GL application that attaches to a 600+ table Informix 7.1 database. I am very interested in talking with other individuals about their experience with scaling this product up beyond 500 concurrent users on a single node system. (database and application on same network node) To date I have not found anyone that has made an attempt to scale a 4GL application to these numbers, and am interested to hear about other experiences. Any insight would be greatly appreciated.