>From my perspective a large proportion of people in the UK have at least one 4K TV, (unfortunately) most of which are 'smart'. Carrying apps for Netflix, Prime, BBC etc.
Having a 4K TV with Netflix won't give you 4K Netflix. You need to actually pay extra for it, about a third more.
If they’re sharing logins, maybe. I bet most families that manage to stream four things at once almost never try to do so on a single service at the same time. Can’t you still get Disney+ for about the price of upgrading to 4K Netflix? Or some-ads Hulu.
Of course, that's the point of the account to share logins, you still get your own user with your own lists, recommendations and all that. Though, Spotify handles it nicer where you can add your own account to the family group plan.
Not often, but often enough to become a nuisance with 2 TVs or so, some tablet/laptop for school work/gaming/whatever and a mobile device per person.
Having a 4K TV with Netflix won't give you 4K Netflix. You need to actually pay extra for it, about a third more.