The issue was that I was being stupid
I had a small typo in my rc.local file causing the second line initialising UART2 to be dropped
I now get this output after boot
Last login: Sat Dec 31 03:12:40 2016 from scotts-iphone
debian@beaglebone:~$ su
Password:
root@beaglebone:/home/debian# cat /sys/devices/platform/bone_capemgr/slots
0: PF---- -1
1: PF---- -1
2: PF---- -1
3: PF---- -1
4: P-O-L- 0 Override Board Name,00A0,Override Manuf,BB-UART4
5: P-O-L- 1 Override Board Name,00A0,Override Manuf,BB-UART2
root@beaglebone:/home/debian# ls -l /dev/ttyO*
lrwxrwxrwx 1 root root 5 Dec 31 03:10 /dev/ttyO0 -> ttyS0
lrwxrwxrwx 1 root root 5 Dec 31 03:10 /dev/ttyO2 -> ttyS2
lrwxrwxrwx 1 root root 5 Dec 31 03:10 /dev/ttyO4 -> ttyS4
root@beaglebone:/home/debian#
Next up is using minicom, physically connecting them together and seeing how much of a mess I can make out of that :-)