Dave, if you want to save a little by sacrificing some power/features/customizability of the Nexus 4, the LG Optimus L9 (which is the 'little brother' of the Nexus 4) is currently on sale for $200 (recently on T-Mobile's website, but I believe currently at Target B&M), which *includes* a $50 card of T-Mobile prepaid that you can easily resell for $40+ which makes it a super deal.
Slickdeals link for this deal
If you want a cheap, decently powered, late model Android phone without any desire to root/custom roms/develop this is a perfect phone.
This phone can be unlocked, but you don't have to unlock it if you wanted to use it on T-Mobile prepaid or postpaid, or any of the several T-Mobile MVNOs. For super light user, I am suggesting the Real PayGo plan from PlatinumTel (platinumtel.com). The rate is 5c/minute, 2c/txt, and 10c/MB with $10 PIN good for 60 days, $20/90 days, $30/120 days, etc. (details are on the website).
If your native T-Mobile coverage is good, I think PlatinumTel's Real PayGo plan is much better than what Airvoice is offering (with AT&T network). In fact, for super light user on AT&T network I think H2O wireless $10 monthly is better than Airvoice's $10 monthly, because the expiration date is stackable.
Edit: If I wasn't clear enough, even if you still decided on Nexus 4, I will still suggest you to go with PlatinumTel instead of Airvoice, especially since you're used to T-Mobile coverage.