You need a USB-TTL Serial dongle plugged on the 3 pins Debug header of the OPi4 board. On your computer where you attached the dongle, you run terminal software such as "picocom" with baudrate set at 1500000 baud. You will then see the log when powering the board. If this Android U-Boot allows to stop booting using <spacebar> pressed several times during early boot and comes to a command prompt, you can then do "mmc list" to see the list of bootable devices, you can then select the SD