Depending on how serious you are with processing your images I would advise against using a laptop screen or laptop all together. Better bang for buck with a desktop setup and a lot more upgrade options for later too. Also make sure to get a screen with an IPS panel. The last thing you want is to be editing images that constantly shift in colour when your viewing angles change. Also if you're set on 4k then I'd suggest at least a minimum screen size of 30 inches anything smaller is pointless for reasons explained in previous posts. Another thing to consider again depending on how serious you want to be with processing is getting yourself a hardware monitor calibrator. It'll take readings not only just from the screen but your environment too. This way you get a calibration that's tailored to your individual workspace. If you want to go next level then create a neutral room by painting the walls grey and use controlled lighting. Otherwise only process at night or keep your blinds closed and live like Dracula.
