Thanks for your message! Actually, it turns out that U-Boot is the source of the problem. I erased the SPI flash and now, when booting directly from eMMC, UART4 does get initialized. Here is the new output from armbianmonitor -u: http://ix.io/2zIr.
When connecting to the serial port though, I get some garbage output instead of the ping messages I'm supposed to get. The baudrate is matched on both sides, what else could be wrong with my settings?
