Depends on the quality. YouTube 4k is about 25mbit/s, so that’s 3-4 4k YouTube videos playing at the same time on a 100Mb/s connection.

4k Blu-Rays OTOH can be about 50GB or larger even. You wouldn’t ordinarily stream that but you could stream one or two blu-rays with a 100Mb/s connection.

100Mbit/s is plenty for current use-cases.

Is a 4k movie really 50GB?

I have a number of movies (about 100-ish titles) in my library that are well above 50Mbps.

Back to the future (1989) as an example is 72.24 GB in my library.