As for myself, think of this: You have a group of 3, with 6 men each, and you go to take on a Super Alien! You send Group 1 to attack the alien, Group 2 as back-up, and Group 3 guards the Skyranger.
When most of Group 1 gets killed-off, you think to yourself: this is one tough alien. Then when the Super Alien goes to attack and kill-off Group 2, cold sweat dripped from your hands as you think: is there even a chance I can win.... and as the last man is about to die.... you.... somehow did it! you won. With only one last soul. And you think to yourself: I hope I never meet the Super Alien again.
That's what I want to happen! I want a Tank-alien to kick my butt to the moon so I can fear it the next time I encounter the thing. And you can't have that with a 4 man squad.
As for those who think we can still have that with 4-6 men, no. Because we get bonus experience for keeping your men alive. So, in other words, the game is telling you that your not allowed to get your 4 man squad killed. And it's easy to reload to save your men.
Besides, remember how useless hero units were in StarCraft? That's because you knew you couldn't let them die, because it was game over. Do you really want to be baby-sitting your more powerful squad-mates like that?