Apple Hardware Test does not support this machine

That downloadable AHT image only supports older Powerbooks. In the description where you downloaded it from, it lists which Powerbooks it runs on. With a newer Powerbook, you need the original OS X disk to run the AHT. Or, if you have either of the TechTools programs, they do the same testing. If you have a newer Powerbook and don't have the original OS X disks that came with it, you can call Apple support, and if they still have them in stock they can sell you a set.

Hi, Brian. There were 23 different Powerbook G4 models in all. The ones for which that downloadable version of AHT is suitable were the first eight, of which the very first was officially named "Powerbook G4" — pretty imaginative, eh? That's what was referred to on the download page you visited, not the whole series of Powerbook G4 models. The AHT versions required by all later PB G4s are not available for download at all, as far as I know.

Yes, certainly Apple's model-naming practices could use some improvement and clarification. But they've been quite confusing at least since 1993, so that state of affairs has become something of a time-honored corporate tradition despite serving no discernible purpose or interest, and I don't expect it to change.

Very helpful information -- thanks. I am also stuck in that I have a PBG4 that is not suppported by the "Powerbook G4" hardware test.