I think it really depends on your gear/equipment and your own eyes/ears. Every setup is different (not just in screen and speaker quality but also in internet speeds and streaming apps). Likewise every person is different in what they’re used (what their quality standard is) to and what they notice.

Streaming 4K can range from awful to very good. 4K Blu-ray will look and sound leagues better than awful streaming but might look nearly indistinguishable and only sound slightly better compared to very good streaming and not overly picky eyes/ears.

The best is to test for yourself. I’d say if you’re unsatisfied with the quality you currently have it’s worth exploring, but if you’re already really content with the quality you currently have then maybe it’s not worth the extra cost?

The iPhone doesn’t have an adjustable aperture. There is no change to the amount of light being let in.