I have a 4k 27'' philips monitor and I use it for my UX/UI work and it has a perfect image quality... as long as I use the best setting:
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Maybe "perfect" image quality for you. But a 4K 27" screen is not proper 2x Retina resolution and does not directly scale the UI elements of macOS. That's a fact, not a matter of opinion. I guess it doesn't matter to you, which is fine, but it matters a lot to many people.