How old are they? Depending on their age, they could both be pullets. Or, they could both be cockerels. Polish are one of those breeds that are tricky to sex. It might help if you posted full body shots of the birds standing in a natural position and a picture of their combs as well.
At 4 weeks it's too early to tell. Could you get a pic of them stood up so you can see the tail? At 4 weeks it would be only guessing as they make look like pullets but turn out to be Roos. I have a shed full at the moment over 10 weeks and some Im still not sure of. Some can take a long time to sex.
Your pic above could be a pullet. It's crest does look a little scruffy but at 4 weeks old it still has a couple of moults to go through before it gets its full adult feathers.