Q: cbs all access

Yes, the CBS app on the Apple TV generation 4 does support CBS All Access.  You can subscribe through iTunes or sign in to your existing subscription in the app. 

The CBS Access app is currently unable to log in registered and paying users. I would recommend not signing up until this issue is fixed. You'll know whether the issue exists because you will be unable to sign up or log in using the app. Until that works, don't bother paying or signing up with CBS separately. You won't be able to access the content on Apple TV until app starts working again.