Check out the first example of the iPhone 6s 4K video capabilities, as expressed by RYOT in its short 'The Painter of Jalouzi'.
According to PetaPixel, a media company named RYOT received iPhone 6s Plus phones in advance of a documentary short production on location in Haiti. The short, The Painter of Jalouzi, is about one man's quest to transform the slums into living art by painting his town in vibrant colors.
Here's the short in its entirety:
While the iPhone 6s can't compete as a 4K shooter with cameras costing several thousands of dollars (especially in terms of being production ready), it's an impressive little still and video camera that you can have at all times for a reasonable amount of money, not to mention its advantage of being attached to an industry-leading smartphone.
Also, check out this behind the scenes video that shows off more pretty footage and reveals the RYOT team's production process, including the use of DSLR lenses on the iPhone 6s Plus.