I haven't tried XM4, but the noise cancelling on XM3 had a very uncomfortable "sucking" feeling even though it is very strong, especially on a plane. The AirPods aren't as bad about this and NC is about as good.
The Sony WH-1000XM4? I'm still on the Bose QC 35, but you got me curious. Could you explain how using it as a hearing aid works? I searched the web but couldn't find good references.
There is a noise canceling mode that let's you hear what is received by the microphone array and an option to "filter in voices while suppressing noise" (they call that voice passthrough). That section of the app is called "Ambient Sound Control" if that helps your research.
She was then able to hear our conversations even though the xm4 are not as good the real thing.
She didnt want hearing aids before that, but afterwards she wanted ones.