You are running 4200 with 4 people. Do you usually have 4 people in the Boat? If fewer, you might over rev. If you have 2 or 3 in the boat and a Tube, you still be fine. A 17 might be better when tubing, if you have 4 or more in the boat.
The 17, 19, and 21, were they 3 blade or 4 blade? Myself...