Also, roosters are lighter colored than the pullets
99% of the time, that's true, but there are some exceptions to that rule, my Lexie being one of them. Still, check the legs-is there a dark wash of color down the front of the legs? Pullets have darker legs than cockerels, in general.
Don't know if it will help any, but that second boy at four weeks then at six weeks was already looking very roosterish, as you can see in this pic. The BRs are almost always known by four weeks old, if not by two weeks even. In that first pic, look at the pullet in the background-see how much...