The first case was reported in the forum (
here) a month and a half ago; the second was from a support email around the same time.
Raze.. THANK YOU SO MUCH !! You are awesome. I couldn't (originally) find any reference to what you mentioned, but you are correct ! Your advice completely worked.
The presence of a Main_1_copy.pak file in the directory in addition to the modified Main_1.pak file was crashing the game. With the original Main_1.pak in a separate directory, the game works flawlessly for 4 players now (after editting the Main_1.pak)
I cannot thank you enough !