The fifth-generation Apple TV, which might be out later this year, could come with 4K support. Bloomberg reports Apple is testing a new unit that can stream 4K Ultra HD content. The current prototype is known as J105.
Apple's next set-top box also reportedly offers more "vivid colors."
The fourth-generation model didn't include a 4K-ready chip due to the costs associated with the component. Adding that chip would have reduced the device's margins.
Simply adding 4K to the Apple TV may not make it a significantly more interesting device, especially given the range of streaming options now available.