You should be working with an array if you're 4K (or serious 2K or 1080P). Individual drive sizes aren't really an issue as much as throughput (the real elephant in the room is back up and archival). Individual drives aren't going to support much more than compressed formats. This isn't something you're going to effectively build internally and never has been. Internal arrays of a couple drives aren't really meant for, or ideal, for this.
So this begs the question, should you even be going 4K? Odds are the realistic answer is "no" and so you're creating problems for yourself that already have solutions for those that actually need it, which is almost nobody except big event, commercial, theatrical. Regardless of what the churn in consumerism tells you you need.
hint: maybe one in ten, if that, blockbusters you've ever seen has been finished at anything but 2K. Sony and the rest are lying to you.