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 nvidia-bug-report.sh 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.
https://devtalk.nvidia.com/default/topic/1043347/announcements/attaching-files-to-forum-topics-posts/

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