I cannot tell by just looking at them. Listen to them and see if any of them are starting to crow. And behind there, toes on the back of there legs are there spurs? You will know when they start to lay too.
Can you post photos of the head in profile, and "head on" as well showing any comb develpment? The brown ones photo is blurry and too small to get a good idea. From what I can tell on photos you have posted, the third and fourth one look like pullets so far. More photos would be helpful and will also give you more responses. Pretty birds. I love silkies!
Helps also to post photos in "full size" option.