Hey!
There is no way for a 4th player to spectate in-game using anything existing in-game right now.
I’m assuming you all are console players, but with crossplay, if even one of you has a PC, it’s possible for that person to stream the game via something like Discord. The other 3 could also be in that Discord channel and the person who’s rotating could watch the stream via Discord and rotate that way.
If you have the capability to stream via Twitch from your console, that’s also another possibility