I think Scrambles are all about putting. Let your "D" player always putt first. Tell her that her job is not to make the putts but to get it close enough for a tap-in or at the very least show you the speed and the break. You'll take a lot of pressure off her and you may be surprised by how many she sinks.
Every team member needs to have a job. The "D" player's job is to "show you the way." And every time one of the other three makes a putt, give full credit to your D player. "I would have never make that putt if you hadn't shown me that break!!"
For other shots the order really depends on the personalities of the players. Usually (and hopefully) your "A" player thrives on pressure. If that's the case they should always hit last. There's no pressure like having three balls before you out of play in a Scramble. If you have someone who's always in the fairway but not the longest hitter, let them hit the Tee shots first. Their job is just to put it in the fairway so everyone else can "go for it."
And try to pick each players ball every once in a while. If your "C" player hasn't been playing well, and she gets a hold of a drive but is 10-yards short of your "A" player, choose the "C" players ball. Tell her you have a better angle from her ball or "it's the perfect distance." Remember, the longest/closest ball is not always the easiest shot.
So, get everyone involved, give everyone a job and take the pressure off on the shots where they can't help out. The beauty of a Scramble is that you have three other players to back you up. Everyone should be relaxed and having fun.
Good luck and good golfing!