In that case, I am fairly confident that the second chick, and the fourth chick are pullets. The first one and the last one, i'm not too sure about. The third one looks like a cockerel.
The main way we have been able to tell is by looking at the feathers on their head. Spiky feathers tend to be cockerels, and more rounded* feathers are pullets. With that, pullets can have crest that are more wide than they are tall, and cockerels can have the inverse. Attached are some pictures of ours for reference. I don't have a good picture of our hen when she was little, but I do have some of our cockerels.
If you want a definite answer, I would wait a few weeks so that their crest will have some more time to develop.
This guy was a bit over a month old, and you can see the crest is very spiky.
These guys were a few weeks old when the picture was taken, both are cockerels.
Found this picture online of a white crested black pullet. Her crest is more rounded, and a lot more wide than it is tall.
They're too young to accurately sex, but I'm inclined to say 1 and 2 are cockerels. They are also not all Polish (not sure if this is a question or not,) but the last looks like a Sizzle which is a non-silkie feathered, but heavily influenced by Silkie genetics, but that's frizzled too. Likely Frizzle Cochin Bantam x Silkie