In aperture priority - set the lens wide open - choose a scene with objects at different distances - now press the button you have assigned to "PREVIEW DEPTH OF FIELD" and the effect should be obvious.
(I checked it with X-S10 but X-T4 should be much the same - its not something I ever use as half press will do the same and also focus on the subject)
Why is there a "PREVIEW DEPTH OF FIELD" function? I am ALWAYS seeing the effect of the DoF. As I turn the aperture ring I see more or less blur on things not at the focused distance. In fact, if I turn the camera round, I also see the blades of the aperture stop move in and out as I turn the ring.
What does the function change? Seems like nothing, as far as I have been able to notice.
If it matters, this is on an X-T4 with a Fuji 35 f/1.4 lens.
Interesting that two actual owners of the X-H2S should have similar thoughts on its IQ, (and the plethora of improvements in the camera itself), and have been able to physically compare it to the X-T4, side by side.
Interestingly the OP, whilst commenting on Ken's review, stated that he had, "already drawn pretty much the same conclusion about it". If you own one and an X-T4 then your conclusion is valid. If not then.......
As I alluded to earlier, I have never seen any photo examples to confirm Ken's points and until then I will treat his reviews as I have always done, which is one of idle curiosity.