I have tested the RTC wakealarms, and I find it works.
Use system image, rockpi4b-ubuntu-bionic-minimal-20190104_2101-gpt.img
Install and update these packages from left to right: rockchip-fstab, rockchip-overlay, u-boot, kernel, rockpi4-dtbo.
And install mate desktop (you can install other desktop)
sudo apt-get install ubuntu-mate-core
sudo apt-get install ubuntu-mate-desktop
To test rtc wakealarm, execute commands at the terminal
$ date -d "today 20:26" '+%s' | sudo tee /sys/class/rtc/rtc0/wakealarm
Click ‘suspend’. Or
$ echo mem | sudo tee /sys/power/state
After the above operation, the red system led and desktop is off.
When it is 20:26, the system runs again. Red led is flashing and desktop is on.