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.