The single player is more about survival, managing your food and water and trying to explore the world on limited oxygen. Most of the overworld is covered dust which limits how long you can stay out exploring. It has some State of Decay type base building type stuff and you'll find survivors out in the overworld to rescue and bring back. You can assign those people to different teams (just like MGSV) and it increases the effectiveness of your farms, exploration team, medical, etc.
The 4 player co-op is tower defense/horde mode. Your single player character and gear is what you're taking into the MP horde mode. Any blueprints and rewards you get in MP are taken back with you to use in single player. It seems like it is worth it to jump back and forth between single and multi occasionally for gear and resources.
I'm weirdly fascinated by this game especially the horror aspects. It's probably going to bomb and get horrible reviews but it has some interesting pieces and controls well (from what I played of the beta).