Just to be clear, while the V4 posts they put in support 1000 volts, the charger they are hooked to are still V3, and are only rated to 500v. We haven't seen a V4 charger that supports greater than 500 volts yet.
I'm no expert, but I have a feeling that V3 cabinets are 1kV capable. The DC bus tying cabinet to cabinet runs at 1kV, so it stands to reason that they can also send 1kV to the charging post (if the post can support it, which V3 can't). Megachargers, from what I can tell, are using V3 cabinets - just more cabinets per post to boost total kW output.