Not all 4k TVs have HDMI ports that support 4k/60. Check your TV's documentation and see if you need to use a particular port. You may want to swap cables or ports with the Apple TV and see if that fixes the problem.
My older Vizio M55-C2 , for instance, only had one port (HDMI 5) that could do 4k/60, the others could only do 4k/30. My current LG C9 has all 4 ports capable of this resolution.