4K streaming needs 15-32 Mbps, depending on compression choices by the service you use, and particularly on the scene content that's being broadcast - fast action and complex imagery doesn't compress anything like as much as most cartoons or non-action sequences. Netflix have focused on "optimisation" of 4K streaming which means that the maximum bitrate would be 18 Mbps, but other services such as Apple TV are sometimes over 40 Mbps.
HD streaming is 5 Mbps (and so usually the most you'll need for Youtube, TwerpTok and the rest), gaming is about 2-3 Mbps, and videocalls/Teams and the like are about 2 Mbps unless they're doing full resolution HD which is uncommon. Audio streaming is typically less than 1 Mbps, as is general internet browsing and social media (adjusting for any cat video content or downloads).
So, if you've got two 4K TVs streaming different content from an "inefficient" streaming service at 40 Mbps, plus two gamers, a Youtube cat video fanatic, and somebody doing a video call, then that amounts to a maximum of about 53 Mbps, to which you need a reasonable amount of contingency. 100 Mbps would do it, but you might find that the 10 Mbps upload is an issue in some circumstances, so in my humble opinion, 200 Mbps with its 20 Mbps upload is perhaps a sweet spot for family connections.
But, your money, your choice. if you want a 1 Gbps connection purely for bragging rights, there's nothing wrong with that at all.
In the strictest sense, yes, 50 Mbps would happily stream 4K content if the connection is working properly. If you were using Apple TV or other services that use lower compression then you'd not have a lot of headroom if any other devices start helping themselves to bandwidth, for example to download updates, but with Netflix that shouldn't be an issue.
But I don't think VM sell new/renewed 50 Mbps contracts any longer, and the lowest you could go with VM would be 100, and if you can take advantage of the Volt offers that would indeed double. Don't forget that any change to your contract (including claiming Volt benefits) starts a new 18 month minimum term, and you can't combine a discounted VM package with the Volt offer. Depending on how good you are at negotiating, it might be better to see what discount you can negotiate rather than taking up a doubling of speed that you'll rarely use?