Which branding are you referring to?
If you mean the "connected carrier" name on the left of the notification bar, there's a setting to only show it if there are no icons in the notification bar, and my weather app (Wunderground) shows the temp and weather in the notification bar all the time, effectively eliminating the "AT&T" branding up there.
If you mean the back cover (I can't remember if there is branding there, I have had the back cover covered over with a vinyl sticker for a long time) new backs are pretty cheap.
If you mean the firmware build itself, I don't think anyone ever cracked the AT&T Note 4 bootloader, so I don't think you can replace the ROM or even root it. I just disable all the AT&T and included apps. The only one I cannot is Evernote.