Sorry? I told someone they could use their credit card to get addition tickets in the public sale and told another person that if they had already purchased 4 tickets during the presale, they had reached their allotment, that no new codes would be issued. Not sure why your code doesn’t have a line through it if you have purchased 4 tickets for the upcoming shows; but 4 tickets is the limit.
From the FAQs:
Q - How Many Tickets can I buy? What is the ticket limit?
A - As a paid-up U2.com Subscriber you are eligible to purchase up to FOUR (4) tickets for any show or combination of shows (for example you can purchase 2 tickets for a show in Brisbane and 2 tickets for a show in Adelaide, or 1 ticket for Brisbane, 1 ticket for Adelaide, 1 ticket for Melbourne and 1 ticket for Sydney.) All ticketing presale code use is audited, and anyone found to be abusing presale privileges risks cancellation of ticket purchases and U2.com subscription.