Motorola just uploaded the source code for the Moto G5S Plus and Z2 Play a few days ago, and now it's the more affordable E4's turn. The phone, which is internally known as "perry," has just had its kernel source put on GitHub.
This is for the smaller Moto E4, not the E4 Plus. The E4 has a 5.0" 720p display, Snapdragon 425/427, 2GB of RAM, 16GB of storage with microSD expansion, an 8MP rear camera, a 5MP front-facing camera, and a 2800mAh battery. And at just $130 unlocked, it's clear that this is a phone geared more for the budget market.
If you're a developer and need the source code to whip up some custom ROMs, you can get it at the source link below.