(Disclosure: I work for Google, but not on the Android team, and in any case, these are my personal opinions.)
You’d think that the 4i is a crippled version of the 4, but it’s not. On the contrary, it’s excellent. It has a similar screen as the 4, which is 1080p at 5 inches.
In fact, in some ways, it’s better than the 4 (watch this video comparison). The 4i has a slightly better screen, with deeper blacks. It works on Airtel’s LTE network, while the 4 doesn’t. In fact, it supports LTE on both SIMs. The 4i runs a more recent version of the OS, based on Lollipop rather than KitKat, as is the case with the 4 . The 4i also has a marginally bigger battery .
The 4i economises on things that matter less, like the material, having 2GB of RAM as opposed to 3, having a 5-megapixel front-facing camera rather than 8, etc. It does not lose any of the critical features of a phone, like the screen or battery or software experience. This is brilliant product design.
This is refreshingly different from most other companies, even Apple, whose cheaper models are crappier versions of their more expensive versions. But Xiaomi tries to make each model as good as it can, for the price. Companies should not deliberately cripple lower-end models of their phones, by choosing cheaper components than are viable at that price range, because they are hurting their own users. Instead do the best you can at every price point. If you make a ₹10K phone, make the best 10K phone you can. If you make a 15K phone, make the best 15K phone you can, while keeping your margins. And that results in the lower-end model becoming too similar to the higher-end model, then get rid of the higher-end model, simplifying the product line, and making it easy for users to pick a model.
When a company makes a good model and a mediocre model, users who buy the mediocre model will be unhappy with the company, and not be as inclined to buy another of the company’s product as they would be if each model were great. And if sacrifices need to be made to keep costs down, sacrifice things that matter the least to the day-to-day user experience. Everyone can learn product design from Xiaomi, even Apple. This is an incredible accomplishment.
 The Mi4 is 2014′s flagship, and the 2015 flagship, Mi5, is not out yet.
 Not that you’d notice it, since it’s heavily skinned.
 It’s 3120 mAh, as compared to the more expensive model’s 3080 mAh. Now, that’s only a 1% difference, so it doesn’t matter at all, but the point is that Xiaomi did not skimp on the battery.