Sony's Alpha A7s Mark II is a full-frame mirrorless camera that's optimized for 4k video, particularly in very low light. Sony's essentially taken the 12 Megapixel sensor of the original A7s Mark I and slotted it into the body of the A7r Mark II.
Great quality 4k video; Clean up to 6400 ISO and still good at 25600 ISO, Capable of shooting in extremely low light at up to 409600 ISO, Very good 1080p options including APSC crop and 120p slow motion, S-Log 2 and 3 for subsequent grading.
Contrast-based AF not great at tracking moving subjects; No APSC / Super-35 crop option for 4k video; Built-in IS only provided one or two stops in my tests; Small dials and buttons.