With a 4K monitor, you'll have to do 1.5x scaling, which will give you 1440p effective resolution.
It's sharper than 1X (obviously), but not quite as sharp as 2X. The problem with using 2X with a 4K monitor is that you get a paltry 1080p effective resolution.
Most people are fine with 4K, 1.5X, 27" monitor and the number of monitors in this sector proves that.
Me personally, I want 1440p at 2X and that's only possible with 5K. I've always wanted a standalone 5K monitor ever since the first 5K iMac, which is why I went with the LG 5K. It has its fair share of issues, but it's worth it for the PQ.
Next up, 8K 40" retina. 😀