I am in the SF bay area, but I am on T-mobile. While I can't speak specifically to the speed you will get on AT&T's HSPA+ on your Nexus 4, I do know that AT&T's HSPA+ is not dual carrier, which means its theoretical speeds are limited to 21 Mbps, while T-Mobile's network, which is an HSPA+ 42 network, can theoretically give you twice the speed. Will real life speeds be different based on this? Yes. I also had the Galaxy Nexus on T-mobile, and I averaged around 7-8 mbps. With my Nexus 4 on the same exact network (T-mobile, SF bay area), I average 12-14. So your phone is capable of getting about twice the speed it is getting on that same network had it supported HSPA+ 42.
Just out of curiosity, given that T-mobile has excellent coverage in the SF bay area, why did you go with AT&T?