Os4 doesnt need amiga hardware, on the contrary it depends on standard hardware and has been particularly ported to support amiga chipset. If anybody wanted it would be easier to use plain ppc emu alone instead trying to fit it within uae.
Adding it to uae makes sense as there are old amiga software that uses the ppc as well. For running OS4 in it's best configuration then an amiga probably isn't the best choice, but it's likely to be the easiest.
There is no such thing as "standard hardware" even in the PC world, you either make hardware that is compatible with some hardware that there is already a driver for or you write drivers to make the operating systems work with your hardware.
Whatever hardware you emulate it's got to be one of the specific configurations that os4 already runs on. Unless you're also going to start hacking os4.
Making a ppc emulator that interfaces directly to the hardware would be interesting, but without an operating system and an abstraction layer then you'll suffer the same lack of drivers that plagues hackintosh and to a lesser extent Linux.
