Season 4 - and it is fun again! (minor spoilers)
I really liked the first season - 2 & 3 were just ok and a little boring at times.
But up until now (episode 3) the fourth season got me again - the small reset helped and it's finally exciting again. Zona was fun. It went downhill a little bit to fast. I guess they have to go back there to find the "Reset" Button machine.
The invisible threat? Nano bots is my guess. They vanish the living and keep the dead longer moving.