From what I can tell Apple TV 4K already support Apple Lossless.
The new thing is that streaming Apple Music will also support Apple Lossless. From Apple's support documents it seems that the Apple TV 4K will only support up to 48 kHz sampling rate.
Honestly, the highest lossless format Apple will be providing seems like a complete waste to me (24 bit, 192 kHz). There is no way anyone will be able to hear the difference between CD quality and that.