The Xiaomi Mi 3 bets on a polycarbonate unibody, more than just a nod to the Nokia N9. The successor Xiaomi Mi 4 is a clear attempt at refinement, with a dash of metal in the mix, and manages to noticeably reduce the overall footprint.
Even though the Xiaomi Mi 4 has the same 5" display as the Mi 3, it's more compact, with visibly slimmer screen bezels. The Xiaomi Mi 4 measures 139.2 x 68.5 x 8.9mm - half a centimeter shorter and narrower than the Mi 3. The Xiaomi Mi 4 has gained some weight compared to the Mi 3 (to a total of 149 grams). It's only 4 grams worth of a difference, hardly anything you'd notice, but the solid feel is there.
The Xiaomi Mi 3 is more subtle in its interpretation of the original and looks and feels like the flagship device it is. One you will easily pick up in whatever lineup of devices you are asked to choose from. It is well-made and likeable, thoughtfully designed with usability and style in mind. Its rear finish is smudge-proof unlike the Mi 4's glossy back. The pitch black front contributes to the premium feel. You can barely see where the IPS display ends and the frame begins.
Xiaomi took a U-turn with the Mi 4 design, dropping the polycarbonate unibody in favor of a metal frame with flat sides a la iPhone 5s, complete with a rear cover that's hard but not impossible to remove. There are as many as 17 clips holding it tightly in place and you would need a small suction cup to pull it open.
Depending on where you reside, you might have an option to purchase some extra rear covers for the Mi 4, which have the texture of wood, marble, bamboo, textile or leather. Those will definitely made your Mi 4 look unique compared to whatever smartphone you place it next to. Unfortunately, details are sketchy on the markets where those will be available.
Xiaomi sticks to the old-school capacitive control deck below the display thus not sacrificing screen real estate. Both phones have the older arrangement with Menu, Home and Back keys.
The Xiaomi Mi 3 features a 2MP front-facing camera, enough for HD video chatting. The Xiaomi Mi 4 makes a splash with an 8MP wide f/1.8 front facer with much higher image quality.
The left side of the Mi 3 is bare, while the Mi 4 has the microSIM slot there requiring a SIM ejector pin to open.
The Xiaomi Mi 3 and Mi 4 have identical right sides - there we find the volume rocker and the power/lock key.
At the top, the Xiaomi Mi 3 has a 3.5mm audio jack, a secondary microphone and a miniSIM tray. Moving on to the bottom, we spot the microUSB port right next to a huge grille hiding the loudspeaker and the primary microphone.
The Mi 4 also has the 3.5mm audio jack up top but only an IR blaster next to it which can, with the right app, turn your Xiaomi Mi 4 into a universal remote. The bottom is where the microUSB port, the primary mic and the loudspeaker grille are.
Around back, the Xiaomi Mi 3 has a 13MP camera lens and dual LED flash. The Xiaomi Mi 4 offers a 13MP camera as well but with a F/1.8 aperture (as opposed to F/2.2)and a single-LED flash.
Both batteries are non-removable. The Xiaomi Mi 3 has a unibody shell and you have no access to the battery whatsoever. On the Mi 4 you can eventually undo the rear cover and see the battery, but it turned out glued in place and we didn't dare pry it away to see whether it's soldered or not.
Winner: Xiaomi Mi 3. Never mind is slightly larger footprint, we just like it better. The Mi 3 is way more subtle in its impersonation, while the Mi 4 is trying too hard to be iPhone-like. The exchangeable rear covers are a point in favor and a replaceable battery would've tipped the scales the other way for sure. At the end of the day though, the Xiaomi Mi 3 is the better looking device, with smudge-proof finish and Gorilla Glass 3.