Can I use the G4 without windows. I use windows all the time, at work, at home, as a HTPC and I know that I can't trust windows. You have to restart it, it crashes etc. That is why it would be good if I could use it with the hardware while it is restarting or starting.
It is a computer, and there for needs an os. Which os is up to you, but don't expect to find drivers &/or support for much beyond XP [not that they don't exists].
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All installed, and ready for visitors. Infill G4 in a 2007 Saturn VUE
If that's what you want... you should keep a traditional headunit (be it factory or aftermarket) and build a CarPC in addition to it. Feed the audio in via aux. That was you always have a working AM/FM/CD player plus the benefits of a CarPC.