Any time a single squad dies in mission 4, I lose the mission. This is not one of the objectives. This happens with or without a Kohan in the squad. I have put a couple hours into doing this mission three times and it happens every time at the last battle where the enemy spawns a huge army and mows through. I know I can beat that army if I can pull back in time but their faster squads always catch up too quick for me to get into defenses properly, and they seem to instantly wipe out an entire squad as soon as the battle starts. In my last attempt, I had the game speed down to minimum and had my army pulling back and it still happened. A squad all died, game over.
I haven't installed any patches beyond what comes with the Steam installation and I use no mods or cheats. Cheats wouldn't help that situation, anyway.
How did you get past that segment of mission 4?