Engine OS v3.3.0 for Denon Prime 4 & Prime GO with OpenSSH server installed and configured.
Flash this onto your device by either using the provided USB-flashable image (.img) or the Windows self-extracting EXE (.exe).
Log in to your device by connecting it to a network via Wi-Fi or LAN and use username root and password denonprime4 (yes, it's exactly this for the Prime GO as well as it's a hardcoded value at the moment).
This unofficial image is not provided to you by Denon and is made available to explore the device for debugging and research purposes. Do not flash this for permanent use.