Best value of well known brands perhaps. You get various Chinese brand ~5" phones with 1920x1080 resolution, 16GB ROM for that price or less now. And those are typicall dual sim, and take 32GB SD-cards too.
Build quality for most of them are probably not as quite high as the Nexus 4, but the one I have at least is very well built.
4.1. No upgrades, and I don't expect any, though there are custom roms (haven't bothered, though I have rooted it - most of these phones are beyond trivial to root). Essentially I'm expecting to upgrade every year, as e.g. in a year it's gone from about $230 for my dual core phone with 1280x720 to about the same for a quad core with 1920x1080 with more memory etc., so I don't really care if there are few or no upgrades for the phone - the reduction in my contract compared to getting an expensive brand phone easily pays down the phone in 6-8 months.
If you intend to keep a phone even at this price range for more than 1-2 years, you'll want to do your research both regarding build quality and upgrades. Though the brands with regular upgrades and "premium" build quality, like Xiaomi, are also pretty much as expensive as e.g. Samsung.
Nor are any available with 2G DRAM, which is a big part of the N4's great interactive performance.
In fact, if you limit the search on that site to 1G phones with Jellybean and 4.6-5" displays, you get a lot of units with prices significantly higher than the new price of the N4. And the ones that are cheaper seem pretty universally inferior (e.g. 480x800 screen, minimal internal flash, etc...)
Tons of resellers on aliexpress.com. Note that they are shipped direct from China, so reading feedback is essential. You can also find some that sell via Amazon marketplace.
Who cares? Part of the reason why I've taken to buying these is that they're cheap enough that I can justify upgrading once a year or so.
(But yes, don't expect to get much in terms of upgrades from the manufacturer. Your mileage may wary, but the ones that do upgrade regularly, like Xiaomi, are also expensive high end brands - Xiaomi wants very much to be the "Chinese Apple" for example)
Here you can find other name for searching (no affiliation whatsoever with this shop, I don't even know if they sell in US, it's only the place where a couple of friends bought their chinese smartphone):
http://www.grossoshop.net/Smartphone_Dual_Sim_Android_Proces...
Build quality for most of them are probably not as quite high as the Nexus 4, but the one I have at least is very well built.