Any G4 Mac can't boot 9.0.4, but Mac-on-Mac can [..]
Uh... no? Almost every G4 can, even the MDD has been coerced to boot it (and Mac OS 8.6, even). There's even a Mac OS 8.6 special version precisely for the G4-based Sawtooth, and naturally 8.6 predates 9.0.4, which has even better G4 support.
[...] also any Mac can't boot System 7.5.2 except early Macs, but Mac-on-Linux with G3 and G4 hosts can do it.
Some people modded some Old World ROM Macs to use G4s and boot into System 7. Also, ANY Mac, even Mac OS 9, can run Mini vMac, which is much faster and more lightweight than Mac-on-Mac, to boot System 7.5.2.
Also... weren't we talking of Mac-on-Mac instead, since this is the Panther thread? Mac-on-Linux on GNU/Linux systems is fine and all but... IMO is offtopic.
However, and speaking of offtopic, since this was touched upon, I must note: Mac-on-Mac, Mac-on-Linux, Mini vMac and EVERY other emulator, they do not
boot into any of those systems per se. Or even if we argue they do, they 100%, without exception, perform
worse than native booting (minus disk I/O, in some cases), even compared to slow Macs, by nature: they introduce input lag, framerate inconsistencies, a cap of either 30 Hz or 60 Hz refresh rate (I use 75 Hz on my native Mac OS systems on a CRT, for instance, which is much smoother than any other monitor tech), and so on. CPU emulation isn't so much an issue, contrary to myth, but overall system virtualization/emulation is. It's not the same as actually booting into the systems, and never will be.
Still, emulation is both cool and useful. Just not a substitute.