That is a good question! My experience is when I have used my presale code, that I have been able to get tickets through the general sale. Remember the ticket limits are per event.
If you are looking at a second show, you should be able to get tickets in the general sale for that event. You will only be able to buy one ticket through the presale because you have used 3 of your 4 allocated tickets.
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 New York and 2 tickets for a show in San Francisco, or 1 ticket for New York, 1 ticket for San Francisco, 1 ticket for London and 1 ticket for Rome.) All ticketing code use is audited and anyone found to be abusing presale privileges risks cancellation of ticket purchases and U2.com subscription.