I don’t know if it is “UltraFine” or not, but I bought a LG 27UK850-W 27" 4K monitor with a USB-C connector to use with my new (last summer) 27" iMac. It doesn’t calibrate quite as well but they are close and I can’t actually see the difference in resolution (both are running “looks like 2560x1440”). The built in speaker of the iMac produces better sound. The one Apple sells is nicer but more expensive.