They made the mistake that many coop games do. They focused too much on balance and made 3 classes way too similar. I would much rather have 4 very different classes that are a broken then a 4 that are balanced. You can fix unbalanced you can't fix bland.

Check back if they add some new classes or a mechanic rework.

UPDATE:
I'm back after the update things are looking alot better.

They made the mistake that many coop games do. They focused too much on balance and made 3 classes way too similar. I would much rather have 4 very different classes that are a broken then a 4 that are balanced. You can fix unbalanced you can't fix bland. Check back if they add some new classes or a mechanic rework. UPDATE: I'm back after the update things are looking alot better.