Resolution is the same it has always been, they are just offering a higher bitrate stream with premium (except I think they offer 8K streams on premium? But like come on… who the fuck needs that).
Doesnt matter if the video is uploaded in 4K, you can watch that stream at 1080p with the same/better effect.
Yes, which is why extra bitrate is valuable (depending on the video content) and the higher the resolution the video is, the higher the bitrate is. Which is why it is beneficial to watch a video at 4K even if you only have a 1080p screen.