Thanks, MichaIng. After installation, I had to use the armbian-config utility, and under System -> DTC, an editor is opened to edit current.dts. I2C4 is listed as 0xff160000, and in the DTF, I had to change status="okay" for this I2C port to work.
Also, I had to re-compile the Linux kernel and set CONFIG_SERIAL_8250_NR_UARTS=4 to be able to use /dev/ttyS4.