I believe you are referring to the bundled Netflix package with Xfinity X1 service?
Even the Netflix app on the X1 has you initially log in with credentials.
Just use the same credentials you use on the app on the X1 box to login to Netflix app on the PS4.
I'm not sure what you mean.
Netflix doesn't offer a package with their streaming service where you can also stream cable.
At least as far as I know.
So your question doesn't make sense to me in that regard.
But to watch Netflix on your PS4 you just install Netflix and run it then sign into your Netflix account.