That is correct, remote sources and/or federation will not work for you.
First setup a trust between the AS JAVAs. Then define the NW2004 portal as an system in the NW73 portal. Choose system of type SAP System, fill only the Web AS settings and configure SAPLOGONTICKET as the Logon Method. The tricky bit is how to create iViews for the NW2004 portal. You could experiment with iView templates on the NW73 portal, as long as you use one that allows you to enter the URL and specify the System, you should be fine. I would probably start with the template called "SAP Remote Web Dynpro iView". Sorry, I have no way to check which one works in your scenario.