they actually tried this once, this what they had to say on it
“The playtests under a 4-1-1 comp were terrible. The problem was the solo support. As a solo support, you felt unable to keep the rest of the team alive. Added to that, at all times, you had 5 very aggressive players prioritizing you as the target every time. As soon as the support player on one side died, the entire team would fall like a house of cards.”
Source: Blizzard save your game! go 1/3/2 - #100 by JeffreyKaplan