Hi. I'm just setting up an edit suite with a new iMac 5k (AMD Radeon Pro 5700 and AMD Radeon Pro 5700 XT) with a second new 4k BenQ monitor (PD2700U). The iMac seems to feed only HD to the second monitor, based on visual perception. Window and text are huge and look really good. Just not 4k. System Preferences shows no alternate screen rez setting for either monitor.
I've tried hooking up via display port on the BenQ to USB-C on the iMac and also HDMI on the BenQ to USB-C on the iMac. The USB-C is going into the thunderbolt 3 port on the iMac.
Any thoughts? I haven't found out much online so far. Only that with the AMD graphics card it will support up to two 4k displays at 60 hz. support.apple.com/en-ca/HT211281
Ha, ha, my brilliant daughter found just the right google search. Crazy, you have to go to the display, click "Scaled" and hold down the "Option" key at the same time. Tada, all the resolutions show up, BUT only for that time. Close "System Preference" and open the "Display" back up and you have to go through the procedure all over again. Is that crazy. Maybe it's only an iMac thing?