kitten question

How old usually before you can tell if it's a girl or boy? My kittens are a little over 4 weeks old and they all look the same to me.

you can tell from birth, they may be all the same gender. there due there first vet check for worming now anyway, he will be able to tell you , or you can post some pictures and if there clear enough we can tell you.
what colours are they?

I can tell from birth, but sometimes they will fool you. If its a multicolor (like tortie/calico) - 99.9% are female and you can use it as a guide.

If they are all the same color then its harder and you might have to wait longer to see if any start growing "balls"