CyanogenMod always does an incredible job of bringing the latest and greatest versions of Android to older devices. The HTC One M8 is still receiving some slow updates here and there from HTC, but will surely be end of lifed soon. The Xiaomi Mi4 is a device that is aging quickly too. With the Mi5 right around the corner this may be anther device that soon loses official support. This is why I have always been an advocate of unlockable bootloaders and roms. With roms you can really extend the life of your device. If you are someone who normally keeps your phone for more than a few years you could probably make good use of a Rom like CM12.1.
CyanogenMod 12.1 is now available for both the Dual Sim variant of the HTC One M8 and the Xiaomi Mi4. CM12.1 brings the latest version of Lollipop to both devices. Installation does require an unlocked bootloader, and custom recovery. You will also want to be sure to install the latest Gapps when you install Cyanogen so that you will have the Play Store and other Google services apps. Grab the rom from the link below.
via HTC One M8 Cm12.1
Xiaomi Mi4 Cm12.1
Gapps