I was able to set screen resolution on OrangePi4 by appending this line:
extraargs=video=1920x1080@60
To /boot/armbianEnv.txt .
On my board the screen is turned on only after a little while after the powering up, so I have figured out that the screen resolution is set by the kernel, not by u-boot. Thus you need to add a parameter to kernel command line.
The line has to be added verbatim without quotes (as suggested elsewhere) and then everything after first = is inserted into the middle of Kernel command line (check dmesg output).
I hope, this helps.