Hi Warick,
Here is what I can share of my experience on the subject.
I just received my X-T4 a few days ago. I'm using it with the new Fuji XF 33mm 1.4 lens as it was explained that those new lenses are a big improvement on the video autofocus part (compared to what the XF35 F2 had to offer, cf the test of Philip Bloom of the autofocus) since it has a linear motor.
I've been testing the video autofocus for a while today. At first I thought that it didn't work all that great, I tried a few different settings but I didn't get a smooth result and sometimes it would not even focus on a new object out of focus that was filling the screen.
While reading the documentation I noticed that the distance limit of focus was 0.3 meters and I had been focusing on objects that were probably in the limit of that range (handheld cam to my face). When I retested the cam and lens and putting myself a bit further than 0.3 meters the autofocus was working really well.
I'll do some further testing and maybe I ll do a video of it if I get consistent results with that lens. I don't own any viltrox lens but I guess I would get the same results as you.
