The old ATV 4k model already supports 4k at 60fps. The only improvement is that it supports HDR 4k content at 60 fps.

From the old spec page [0]:

   H.264/HEVC SDR video up to 2160p, 60 fps, Main/Main 10 profile
   HEVC Dolby Vision (Profile 5)/HDR10 (Main 10 profile) up to 2160p

And the new spec page [1]:

   H.264/HEVC SDR video up to 2160p, 60 fps, Main/Main 10 profile
   HEVC Dolby Vision (Profile 5)/HDR10 (Main 10 profile) up to 2160p, 60 fps

[0] https://support.apple.com/kb/SP769?viewlocale=en_US&locale=e...

[1] https://www.apple.com/apple-tv-4k/specs/