I didn't know that UAE was usable under NT4.
Doesn't UAE demand DirectX 8?
Developers don't bother testing their code on NT4, so go on about DX [whatever the latest version] is "required" - Quake 2 and 3 run perfectly well on NT4, and Quake 3 "requires" DirectX 7... NT4 is capable of DX3 btw.
There's very little that recent versions of DX could offer an emulator. For example, the main additions into DX9 are enhancements for defining 3D structures a little better, not a lot of use in an Amiga emulator :-)