Programming a line key via the handset is tricky but doable, when you configure the key you can also assign the ringing at the same time.
The problem comes if the other 3 lines are ringing to a multiple call group or phantom DN then you will need the eManager software to program the line 4 to also do the same.

If you don't have the eManager software I would get someone to do it for you remotely as it will be much quicker and cheaper in the long term than having a crack yourself and wasting lots of time on it.
Is the system connected to the customers LAN? if it is then get the port forwards configured and it will make it easier for someone to help you remotely.


Regards
Carl