Yes, 4,5k is decent. But put that in perspective of type of data and content Facebook exposed of its billion users.
And personally if Facebook's says "no maximum reward" (https://www.facebook.com/whitehat) I was expecting something different for a complete account access exploit.
Perhaps they didn't think it could be weaponized, so to speak. Accessing one account at a time isn't as big a deal as systematically accessing many accounts. Even if your exploit could do so, they may have safeguards against such robotic access.
And personally if Facebook's says "no maximum reward" (https://www.facebook.com/whitehat) I was expecting something different for a complete account access exploit.