I tried to install Windows NT4, and I gave it the highest available CPU in QEMU (because that's the default, and of course it's an x64 capable CPU), and it crashed after text mode install. It did not produce a normal panic screen but rather it trashed video memory before transitioning to graphics mode.
The QEMU documentation says that NT4 works with a a pentium CPU; which I trust as far as the guy testing it found that he needed to limit the CPU type down to pentium, but I would not be suprised to find it was the only CPU type tried as NT4 wouldn't know how to get any benefit out of a newer CPU anyway.
Restricting the VM to a Pentium CPU resulted in the install working.
Why did Windows NT4 crash on the emulated x64 CPU?