I already have a gaming monitor with ultrawide resolution. So I'm just searching for a decent 4K monitor to watch movies on. I'm looking for 27" as it matches perfectly with the 34" I have.
It would be advantageous if it had;
Samsung 4K TVs are the best options as a computer monitor with fast response. You can google an article about this.
For general office computing, 1440 is sufficient for text clarity.
If your gaming monitor with ultrawide is 3440x1440, then you'll want a 1440 27" screen to go next to it so that the resolutions match up when they're side by side.
I have a Samsung LC34H892WGEXXY next to a Dell 2721DGF and they are great side by side. The Dell surround is black while the Samsung is gunmetal grey, but the screen sizes and resolutions line up well, along with very similar bezels including a fatter bit at the bottom where the controls etc are.
27" is way too small to enjoy watching movies on in my opinion. Why not just use a 4KTV?