This is what Ticketek replied to my query regarding the U2.com terms:
Sydney = 1 show
Brisbane = 1 show
These are separate shows, it means you cant buy more than 4 tickets to Sydney on the 13 Dec, for example, however you can buy another 4 tickets for Brisbane.
This is how this information was confirmed to us by the promoter.
It is then advising you that your code can be entered once to buy tickets.
NO, the ticket agents are incorrect. Its ONE code for up to four tickets for the SAME show. No More - Baja