4 days is early to candle. Wait until about day 10, or at the very least, day 7. Day 4 for is early for chickens, they take 21 days. Ducks take 25, don't they? So it's even more "too early" for ducks.
Be patient. Being excited doesn't make candling too early work any better.
The dark spot you see, once they develop that far, is a good sign. That's not the same as the bullseye you see on the yolk we were talking about. See the sticky on fertile vs. infertile eggs at the top of this section, you'll see what I mean.