A 27" 2K monitor with good color accuracy will suit most usage cases unless you are a professional. The benefit of stepping up to a 4K monitor however, is the ability to see more of your photo on the screen at once. The downside is that a 32" screen is the smallest you should aim for with 4K which takes up valuable desk space and these screens will be much more expensive. It is better to prioritize color accuracy over resolution if you are on a budget. Ultimately whether you need a 2k or 4k monitor for photo editing depends on the size of images you will be editing and how often you need to do so.