Sky only have around 600k homes with Sky Q and not all of those have 4k TV sets so depending on the number of 4K sets out there and virgin V6 boxes iPlayer may have a bigger 4K audience than Sky. There may be more Apple TV 4k boxes connected to 4k sets than sky has Sky Q connected to 4k sets.
Only 600k in 2 years now on Q? Where did you get them figures from?