I got a Dell UltraSharp U2720Q a couple of weeks ago. I'm very happy with it - 27", 4k, DP and HDMI ports, USB-C input. Just make sure your graphics card supports the resolution.
I think you have to be careful about the colorspace if it is for photography. A monitor might be 4k but if it is srgb only I don't think it is really that much better for photo editing. You can always enlarge the image on a HD monitor to see more detail. I think you have to look at the specs for 95% or more of Adobe rgb, If it is wide gamut they will brag about it in the specs.
A tec friend recommend BenQ. Mine work great.
You can get an HP Pavilion 27-34 for under $450. Excellent contrast and color. Best Buy, HP or B&H.
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