Telemann Concerto in G Major 2nd movement Help...

So, I have been struggling with this concerto for a long while now...

Mainly, the part where you play the D-A-F sharp - D quad stop. I can not seem to align my fingers in time, and often "mute" some unnecessary strings. Even playing that quad stop alone is difficult. (I can play the previous quad stop in C just fine) My teacher has been trying to help me out, but nothing seems to work. I am tempted to just use my ring and pinky to play F-sharp and High D since it is easier that way, but I know that isn't conventional/proper.

Any advice? Could I just be playing on a viola that is too big for me? (I am 5'11 playing on a 16.5" currently)