This should be an easy match, unlike the still image quality. The Xiaomi Mi 4 supports 2160p and 1080p video capturing at 30 fps, plus it enable HDR mode in both resolotions. The 4K clips have a bitrate of around 40 Mbps, audio is captured at 96 Kbps with 2 channels (read stereo). The Full HD samples bitrate drops down to 15 Mbps, while keeping the audio quality the same.
The Xiaomi Mi 3 is limited to 1080p video recording only. The clips have a bitrate of around 15 Mbps, audio is captured at 96 Kbps with a sampling rate of 48 kHz with 2 channels (read stereo).
The detail levels in the Xiaomi Mi 3 videos are high, while colors are pretty good and the framerate very consistent at 29fps. Contrast and white balance are great too for some impressive footage overall.
The 4K videos taken with the Xiaomi Mi 4, even though recorded with such high bitrate, look somewhat soft and not as contrasty or clear as the 1080p videos. The colors look good but the white balance sometimes is off and you may need to tweak it manually.
On the other hand the detail levels in the 1080p videos are very high, colors and contrast are great too, while the framerate is very consistent at 30fps.
Xiaomi Mi 4 supports HDR videos in both 4K and 1080p recording modes, which is a first for a smartphone. Even though it rescues fine detail in the shadows and in the highlights, the HDR effect looks rather unnatural and we suggest you use it only in exceptional cases.
Check these HDR videos on YouTube:
The Xiaomi Mi 4 and Mi 3 are available it our video comparison too. There are plenty of 2160p and 1080p camcorders you can compare them with. The Mi 4 might perform average on 2160p mode, but it has proven to be among the best with great detail, colors, and contrast in the 1080p range, beating the Xiaomi Mi 3.
Winner: Xiaomi Mi 4. It offers better video quality, HDR option and higher resolution 4K option.