Bingo. Carriers want you buying their own selection. My local store are not only unwilling to answer even simple APN questions about porting my phone to their service, the reps are downright rude about it.
That sounds totally unprofessional for someone at AT&T to not help you with your Nexus 4. I mean, you're willing to pay money to use their service! Did you ever speak to a manager or contact a customer service representative on their website? It sounds like they owe you an apology.
Perhaps T-Mobile is doing better. They're the only carrier in the US to sell the Nexus 4. I would be curious to see how the Nexus 4 performs in terms of sales with the rest of their product lineup.