According to FCC documentation, the Moto G4 and Moto G4 Plus come with model numbers XT1622 and XT1642 respectively.
Apart from the FCC listing, the Moto G was also recently spotted in benchmark site. The Geekbench benchmark listing tipped that the Moto G4 features a Snapdragon 617 processor coupled with 3GB of RAM, and will run Android 6.0.1 Marshmallow.
Previously, prolific tipster @evleaks had shared a leaked image of the Moto G4 Plus, showing a square-shaped fingerprint sensor placed as a home button, a speaker grille, a front-facing camera, and the rear camera with flash and autofocus sensors enclosed in a pill-shaped module and a dimple with the company logo below the camera. There were two microphones visible, placed besides the fingerprint sensor and the rear camera.
One of the biggest differences between the Moto G4 and Moto G4 Plus is said to be the fingerprint scanner which will come embedded in the home button of the latter while the former will not include any fingerprint scanner.
Based on preliminary leaks, the Moto G4 is expected to feature a 5.5-inch full-HD (1080×1920 pixels) resolution display, an octa-core processor, 2GB RAM, 16GB of inbuilt storage, and a 13-megapixel rear camera. The Moto G4 Plus is rumoured to sport a fingerprint sensor, and a 16-megapixel rear camera.