And, would People of Color actually be watching this show in vast numbers if one of the four girls was nonwhite? Blacks famously wouldn't watch Seinfeld, and I doubt if adding Martin Lawrence to the cast would have done much for Seinfeld's black ratings.
We can actually test this hypothesis about "Girls" quantitatively right now by looking at how many blacks have bought tickets to see Whit Stillman's current movie Damsels in Distress, in which one of the four damsels is black. My guess is that Troy Patterson would go see any Whit Stillman movie no matter who was in the cast, and the vast majority of other blacks wouldn't go see a Whit Stillman movie even if the four damsels were played by Beyonce, Mo'Nique, Lil Kim, and Tyler Perry.
Speaking of George Costanza, that also raises the question of earlier New York sitcoms about People of Pallor, such as Seinfeld, where the only memorable black character was Kramer's Johnny Cochran-inspired lawyer. (I vaguely also recall a black executive who played straight man to George's fecklessness).
After Seinfeld and, especially, after Curb Your Enthusiasm (by which point Larry David had a half billion dollars, so even if he got Michael Richardized if a race joke went over wrong, he'd still have a half billion dollars), isn't it pretty obvious that Larry David's views on race are closer to mine than to those of all the folks who are in a huff over Lena Dunham's three titular friends being white? At least on the central issue -- Race Is No Joking Matter! -- me and Larry are on the same side of the barricades.