I have a 27" 4K and I use it at 1920x1080 although I have tried it at 2560x1440. It's not nearly as crisp at 1440 as it is at 1080. But the general size of things at 1440 is good. I just use it at 1080 and I have Safari in split screen with "page zoom" at 85%. Seems to do okay for me.
I guess using a lower resolution on a 4K Monitor can cause this "unsharpness". I would recommend using the 4K resolution instead.