Issue on P620 cannot display 4k resolution with Linux OS

I have a HP Z8 G4 workstation installed with NVIDIA P620. When I connect it with 4 units monitor (via mDP to HDMI adapter) to form one large screen, it can only display 2k resolution. Checked the adapter support 4k, I have no idea how to make it display 4k resoluton. Anyone has idea pls?

Are those active (4k@60) or passive (4k@30) converters? Which brand/model?
Please add

Option "ModeDebug" "true"

to the device section of your xorg.conf and run as root and attach the resulting .gz file to your post. Hovering the mouse over an existing post of yours will reveal a paperclip icon.

The adapter supported 30hz only. So where can I set it to 30hz inside Linux?

In the monitor manager of your DE, e.g. for Gnome, in gnome control center->devices->displays.

BTW, are you sure those monitors don’t have a displayport connector?

i am connecting the monitor via mDP to HDMI adapter + HDMI cable. thank you