CyanogenMod has released the official nightly build of CM12.1 for Xiaomi's current and last generation flagship smartphones – the Cancro duo of Mi 4 and the Mi 3. The ROM is built on top of Android 5.1.1 Lollipop, which is the latest version of Android for all non-Nexus users as of now, and is available for both the WCDMA (3G) and LTE (4G) versions of the devices.
The builds have been a long time coming, seeing as unofficial builds of the same were being tested for a while now, and were available for the adventurous souls to download and try on their devices. The new ROM is expected to come with a number of new features and improvements to the smartphones, which are currently on MIUI 6, which is based on Android 4.4 KitKat. The Android 5.1.1 Lollipop-based official MIUI 7 build from Xiaomi is expected to hit the devices over the next few weeks. The international version of MIUI 7 has already been debuted by Xiaomi at an event in New Delhi last week, but not all devices getting MIUI 7 will get Lollipop.
Coming to the latest available ROM for the two devices, both smartphones come with the same underlying hardware, which is why they both get the exact same ROM. Sure, the Mi 4 comes with slightly higher clocked CPU and GPU, but that's about it. As for the CM12.1 Nightly ROM, it is expected to bring the look and feel of stock Android with the feature additions and enhanced customization options that we've come to expect from Cyanogen. Would-be DIY flashers however, should bear in mind that a nightly build is still a work in progress and is expected to come with its own quirks and issues. Any major bug of course, will eventually be ironed out by the CM team in the course of time as the nightlies graduate to milestone releases, but as of now, the software is expected to be buggy and there's no guarantee of stability. If that doesn't sound too scary, users of either of the devices can head over to the official Cyanogen repository (source link below), where the firmware is available for download.