EDIT
I called AT&T and had them put me on a 4G plan. It took them a while to finally find a dummy phone that is 4G HSPA+, so if you call, have them use the Inspire 4G so that they can attach the Smartphone 4G Plan.
I can now run the Speedtest and am getting lower latency, meaning I am using HSPA+
I used this APN:
This should be a sticky thread in benefit of ATT users.
Like you mentioned, I had to call A&T after booting up my Nexus 4 and noticing that even though I could browse the web, and download apps via Cell Data, the speeds weren't that good.
And like you, the speedtest app wouldn't run the test after ping.
I had to give a friends EMEI of a Galaxy S2 and they where able to change my Plan from an iPhone 4S plan to a "DataPro 2 GB for Smartphones on 4G" plan.
Still, AFTER changing the APN settings to HSPA+ as showed here is when I was able to run the Speedtest app and my Speeds are a bit higher than on my 4S.
I normally had a max of 9MBPS with my 4S in my home.
Right off the bat I got 13.8 MBPS with the HSPA+ settings.
Thanx!!