If you're in the US, the Nexus 4 doesn't support CDMA, which rules out Sprint and Verizon. AT&T doesn't give a discount for bringing your own phone, so you'd be losing money if you don't take a device for them but buy a data plan. So you're effectively locked into Tmobile.
Postpaid plans are the problem here. They're something of a tax upon people who can't do math if you have a subsidized phone; they're terrible if you bring your own phone.
If you bring your own phone, you want to be looking at the prepaid arrangements. AT&T has considerably cheaper prepaid plans than their postpaid plans (I think it was like $40 cheaper when I looked?). T-Mobile's prepaid plans are even cheaper; I pay $30/month for 100 voice minutes ($0.10/minute overage, which I've hit a couple times), unlimited data (speed-throttled at 5GB), and unlimited texts.
I paid $350 for my Galaxy Nexus and ditched my $90/month AT&T postpaid plan for the $30/month T-Mobile prepaid plan, and the delta in plan costs paid off the cost of the phone (over a carrier subsidy) in about four months.
T-mobile's postpaid "value" plans can also be a good deal (no phone subsidy, but yes contract/ETF). We just moved from the prepaid $30/month/line plan to a family value plan, because we could get 5 lines for <$20/month/line (unlimited talk/text on all lines, 200mb data on two lines).
AT&T doesn't give me a discount for bringing my own phone and Sprint/Verizon went with CDMA instead of GSM, thus the phone is carrier locked in to T-Mobile. I don't know what to make of this. May be magic should've been real - we would get a truly unlocked phone that can be used for free at highest speeds on any network in any part of the world :)
There are pre-paid providers which operate on AT&T networks and offer services (including upto 5GB data) for about $50. If you are not on a family plan, buying an unlocked phone and going pre-paid is actually cheaper.
