You get a Netflix family plan if you want four people to simultaneously and independently connect to Netflix, each consuming their own stream. You don't need a family plan if you plug Netflix to your TV and invite three other people to watch a movie together with you.
Tethering is like the latter case. Your phone is still a single device consuming a single Internet connection. The concept of "multiple streams of internet data" you used exists only because phone companies violate net neutrality, refusing to become dumb data pipes they should be.
You get a Netflix family plan if you want four people to simultaneously and independently connect to Netflix, each consuming their own stream. You don't need a family plan if you plug Netflix to your TV and invite three other people to watch a movie together with you.
Tethering is like the latter case. Your phone is still a single device consuming a single Internet connection. The concept of "multiple streams of internet data" you used exists only because phone companies violate net neutrality, refusing to become dumb data pipes they should be.