I only purchase the ones that bundle both the Blu-Ray and 4K UHD discs together. They end up costing more (as I import Region A than purchasing Region B) since Region A copies by default have the subtitle language (i.e. Japanese, Traditional Chinese) I want while Region B does not have it included since they often focus too much on European languages (i.e. Spanish, French or German). Do you consider 4K UHD better than Blu-Ray?

A blue ray could be mastered better than a 4K release, but if both are mastered well, 4K is literally 4x the visual information. With a 4K display, you’ll see detail not visible in the blue ray release.

Absolutely, if you have a 4k display

Absolutely, especially HDR makes a huge difference, I’d say more than the resolution increase.

I thought blu-ray was 4k? Guess I need to do some reading

blue ray was originally 1080p (but also can be 480 or 720 depending on source material)

If you can get your hands on an UHD-compatible player than yes.