From what I understand they still seem to render at 1080p and then upsample to 4k. Judging by Soul.
it's almost certainly rendered in cinema 4k (4,096 x 2,160 pixels), if not more. moreover it'll be in a 16/32bit log colourspace as well
Have you examined it or are you guessing? Because I'd have assumed they did too but I see jagged stairsteps on the 4k stream when zooming on sharp edges.