You're likely not going to see anything at day 4. I suggest that you go to the learning center and read all of "hatching eggs 101". It's a long read, but IMO, should be required reading before any one is ever allowed to plug in an incubator. I've been hatching chicks every season for 4 years now, and every single season, I review that article! You can tell if an egg is fertile when you crack it open for your breakfast. Again, you need to know what you are looking for, and the difference between a fertile egg and an infertile one is very subtle.
Finally, your birds have just started laying. In order to ensure that your ducklings have the best possible start at life, I suggest that you wait a couple of months, until their eggs are larger before trying to hatch them. Are you in the northern hemisphere? If so, you're heading into winter. Do you really want to be brooding ducklings in the winter? If you're down south, it may be a non issue. However, if you're in snow country, such an undertaking would be brutal. While it can be done, there is no way that I would consider doing so here!!!