Yes, I'm from IBM, and yes, WSAD 5.1.2 supports JDK 1.4, as did WSAD 5.1.1 also. We added support for JDK 1.4 in WAS 5.1, which was first supported by WSAD 5.1.1.
For standalone application, JDK1.4 works, but for J2EE development, it seems WSAD always use the jre that under WSAD_FOLDER/eclipse/jre, even after I had changed current jre to Sun's 1.4.2 jre. Is there any other configuration I need to set to make JDK 1.4 work for J2EE development? My WSAD version is 5.1.2 on Windows.