If you are candling, you will probably see nothing at all. The embryo has very small blood vessels that are just forming and a tiny transparent body all isolated in a circle about the size of a quarter on the yolk. If you are hatching and trying to take out the bad eggs, I would wait until day 7 at least, I usually wait until day 10 to candle and take out bad eggs to be sure. On the other hand, if you crakced one and was wondering how far along it was when it died, there's your description, on the yolk there will be a circle about the size of a quarter with a transparent embroyo in the center and tiny blood vessel's branching out. I know this as I often hatch chicks, but even if you google 4 day old chicken embryo, you will find images. Just make sure they really are 4 days.
