I was able to get a Thinkppad Extreme laptop with an internal 4K monitor and a 1920x1080 external monitor (HDMI-0) running i3wm stably using optimus-manager with the startup mode set to nvidia, as suggested in this forum.
I recently tried using ‘xrandr --output HDMI-0 --scale 2x2’ to zoom out the external monitor. However, after about 15 minutes, the Xserver seems to crash with the internal monitor no longer recognized. I can only get back to the previous stable state by rebooting. This seems similar to a problem reported here: https://bbs.archlinux.org/viewtopic.php?id=222272 but I am not not quite clear on the resolution of the problem. There does not seem to be any obvious errors logged at /var/log/Xorg.0.log.
Have any of you run into a similar problem and how did you resolve it? Thanks.