The Xiaomi Redmi 4A (aka Rolex) is a cheap (~100 USD) and lightweight (131.5g) 5" phone. For price, weight, and size, there is not much competiton. The screen is a 720 x 1280 px 16x9 screen that is pretty decent. Its got a Snapdragon 425 and 2gb of RAM with 16 or 32 gb of storage, and supports dual sim or single sim + microsd card. On the negative side, the camera is weak, it is slow to open, and of course stock MIUI is slow and bloated, along with an annoying reminder to register your sim aka spyware on the prowl. That said, there is nothing in a 5" screen that is as light and as cheap, and that means very strong value. Thankfully this is a popular device for custom roms. Rolex is not as widely supported as some devices. However, the AOSP Extended ROM, a twice monthly updated vanilla Google + customizations does support this device.
Notes as of November 2018
I still prefer this phone over the Xiaomi Redmi 5A and Xiaomi Redmi 6A. The 6A is a bit heavier, with a longer screen, but the home row buttons have been moved onto the display, so no real gain in screen real estate. The 5A is not as nice a feel in the hand, but some versions have 4gb of ram and 32gb of storage, so that is better than the 2gb generally available across all of these models. Overall, a good balance of lightweight, decent screen, moderate performance, and lower-end camera. Hard to beat for around $100 usd.