was 5.0 fp3 has JDK1.4 client support. You don't have to wait for 5.1, which means server side JDK1.4. Your problem is to have JDK1.4 client interoperable with WAS 5.0 server (JDK1.3).
So my suggestion, download was 5 latest fixpack, find readme, and see how to enable JDK1.4 client environment. In the meantime, just develop your EJB client in JB with JDK1.4(if it won't be deployed inside WAS).