External 4K display on T580

[r1ncew]

I own a T480 as well as a T580 which has about the same hardware. Great job!

I spend some time to figure out about external 4k displays and got it finally running on my T580 @60Hz on HDMI as well as USB-C

To Display 4K resolution the BIOS needs to support 64MB DVMT memory

The T480 only supports 32MB which limits external displays to 2560x1440 with the enabled whatevergreen frambuffer-stolenmem hack. There is a posibility to flash the T480 BIOS with an external eepromer and enable this setting but loosing the TPM module. Check it here if you want to risk your T480.
https://programming.vip/docs/thinkpad-t480-unlock-bios-hidden-menu-modify-whitelist-cfg-lock.html

On T580 the 64MB DVMT is enabeld by default as it also could be also configured with an 4k screen.

Just set the BIOS graphics memory setting to 512MB and remove these lines from the T480 config.plist as the T580 detects the correct memory automatically.

framebuffer-fbmem
AACQAA==
framebuffer-stolenmem
AAAwAQ==

I hope this will help some people wo ran into the same problem.