As I read it, this is the parent poster's point. An individual person is unlikely to use Netflix on two devices simultaneously, so a notional "four screen plan" that also charges per profile (as per the linked article) undermines itself.
It says the additional household subs get an extra profile for them, and that there will be facilities to transfer profiles. There's nothing indicating that the account won't still be able to have five profiles for people who live in the household, as is currently the case.
I realize that traditional cable is account per house but that's what streaming services were supposed to break us away from.
Then the streaming is being done by someone who lives in your household and is permitted under the terms and conditions. And will probably cause no issues, as Netflix will likely register that device regularly appears at your IP address with all your other devices.
If you have kids. Which is, y'know, not uncommon. The four screen plan has always, since it's inception, stated that is for four people who live in your household in the terms.