At the moment, I don't believe we have plans to provide a 4-player bundle for Spintires: MudRunner or American Wilds.
American Wilds will require the base game to run, that's correct. You can get both together in the American Wilds Edition version of the game.
Many thanks. I am part of a group of players and we are looking for Multi-player games to support normally between 8 and 24 users, although I have seen spikes of 76 users on our Empyrion Server.
Thanks for the response and will pass on your comments to the group.