There's no reason for there not to be a 4 player campaign (unless it is a technical issue).
If it's simply because "people don't want it", then;
I often wonder why people would want to refuse the idea? If you don't want to play a 4 player campaign, fine, then don't! Me and my friends would love to play a 4 player campaign. It's subjective, you don't have to play it if you don't want to so why complain about it? It's really not a problem to me and my friends of how long it'll take, we can do other things on our computers, it's just better to play a campaign where all of us are involved then to start multiple campaigns with everyone individually.
If it's technical, then I'm not going to tell them how they should do only that it shouldn't be off the table. I'm sure working towards bigger multiplayer options shouldn't be off the table. I'm not saying it'd be easy, I'm saying they should work for it. "Easy said than done", sure, but I don't care when it's released only that it becomes something in the future.
To be honest, the multiplayer options is what's keeping me from buying further into the Total War series, I stopped at Attila. We honestly would only get 3K if it has a bigger multiplayer campaign feature.