You don't say what sort of work you need this for. Nor do you mention a budget. A good display for the average user who wants something which looks clean and crisp is a lot cheaper than what somebody needs for pro video and photo work.
If you want retina pixel density. You'll need a 5K diskplay. As far as I'm aware. The only one left on the market is the LG 27MD5KB-B. Outside of Apple 5K was a flop.
https://www.bhphotovideo.com/c/product/1359254-REG/lg_27md5kb_b_27_thunderbolt_3_led.html
If you don't care about retina density. 4K is perfectly fine. For proper 1:2 conversion. You'll need to set it at 1920x1080 scaled. That way text an UI elements will be the equivalent size of a 27" 1920x1080 display. Just with sharper detail.
You can set it to 2560x1440 scaled. As it is an odd conversion. You do get some minor scaling issues as
@frou alluded to in the above article. Although I set two 27" 4K LG Thunderbolt displays up to an iMac 5K for a client. With all set to the same scaling (2560x1440). The 4K displays are hardly distinguishable from the iMac's 5K display. This computer is being used for office tasks. I couldn't say the same would be true if used for pro work or video. Never tested it.