Android 8.1 Oreo on Galaxy Note 4! [Install Guide/HOWTO]
While Samsung has stopped updating the Samsung Galaxy Note 4 since Android 6.0.1 Marshmallow, you can still manually install your Note 4 to the latest Android 8.1 Oreo by installing a custom ROM if you have one of the models supported.
NOTE: This is an AOSP ROM! (What is AOSP?)