The a6400 killed the a6500.
The A7III killed the A9.
Now, the RX10 IV is looking fuddy-duddy next to its new baby brother.
What's the killer edge?
HLG - The RX10IV doesn't have it.
That, and of course, the sheer size difference. If a 24-200mm zoom range is all you need, it makes more sense to beef up the RX100's audio capability with a custom cage (the ones for earlier version will block the new designed HDMI port) and a small micro HDMI-to-3.5mm headphone dongle, then to carry around a RX10 IV. There is clearly still a significant size advantage to be reaped.
The miniscule size of the RX100 also means lighter and smaller gimbals like the Crane M2 or the Came-TV Spry.