Can’t get 4K on both monitors

I have 18.04 installed on a Mac Mini from (I think) 2015. I just installed two identical 4K monitors, one through the built-in HDMI port, the other through the built-in Thunderbolt port via a MDP->DP cable.

Both monitors are detected and seem to work. The one with the DP connection displays in 4K. The one with the HDMI connection has a maximal resolution of 1920×1080.

I’m a relative newbie and am not really sure how to start troubleshooting this. I’m not even certain if the problem is linux configuration or an intrinsic limitation in the hardware. Help? Thanks…

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Author: grichard