How can I make it so on 4 player coop, the team is split in half and in solo, you spawn in one of the two locations without the zombies spawning on the other side?

Have the same problem 

How can I make it so on 4 player coop, the team is split in half and in solo, you spawn in one of the two locations without the zombies spawning on the other side?

Just make two starting zones with different names.

The only thing that makes the start_zone name special is that it is activated when the game starts, if you can do this with another zone it would essentially be the same except with 2 zones.

The start zone is initialized inside your mapname.gsc, just do the same thing with a different zone to make it count as a separate spawn zone that will only spawn zombies when someone is on that part of the map.

You can also make 1 big start zone, and add traversals so that zombies can go from a side to another, which can also be useful even if you have 2 separate zones in case people in zone A die, so zombies could go to zone B where there are still players alive. 2 separate zones should be better though, since zombies would come faster.
If you need traversals JBird did a tutorial with prefabs you can download : http://www.youtube.com/watch?v=XjbI1qaweYc&t