Xiaomi Mi 3 and Xiaomi Mi 4 rely on Sony-made sensors with identical field of view. The only difference between the two cameras is the aperture size - Mi 3 is set at f/2.2, while the Mi 4 is wider at f/1.8. Theoretically this will help Mi 4 produce better quality pictures in shot in low-light conditions.
The flash of the Mi 4 has also been altered - instead of the dual-LED the Mi 3 features, the Mi 4 comes with a single-LED flash, but bigger in size.
Xiaomi Mi 3's camera resolves plenty of detail. The colors are somewhat oversaturated and the white balance sometimes is off, but overall, the Mi 3 image quality is great. The noise levels are a bit higher than we would have liked though.
Just like the Mi 3, the Xiaomi Mi 4's camera resolves plenty of detail, color reproduction is accurate and the white balance is right most of the time. We noticed little oversharpening here and there (the street lamps), but not enough to ruin the great impressions. The noise levels are above what we'd call standard, higher than Mi 3's even at ISO 100, and that's especially noticeable in the shadows or in the sky.
Despite the higher noise levels, we would rate both the Xiaomi Mi 3 and Xiaomi Mi 4 cameras as excellent in daylight. We would pick the Mi 4's camera before the Mi 3 because of its more natural white balance, the Mi 3 prefers colder settings.
We snapped a sample in low-light conditions and this is where the Xiaomi Mi 4 and its wider aperture really makes a difference. Both shooters use the same shutter speed, but Mi 4 ISO is lower because of the wider aperture and thus the quality of its sample is better. We should mention though that the Mi 4 autofocus in low-light scenes is often off, while the Mi 3 camera gets it right.
The HDR on both phones is conservative and rescues both the highlights and shadows without making the contrast too low.
Also, check out our Photo comparison tool that pits the two phones against each other.
Winner: Xiaomi Mi 4. The Mi 4 camera has an edge over the Mi 3 with the wider aperture and better white balance.