I've heard that but don't believe it to be true, from my experience. In any case it is going to take a while longer to know for sure. Try again when they're 7 or 8 weeks.
It's still early to be certain. But I'd guess pic #1 is a cockerel and pic #3 is a pullet. The other two are tougher to guess on at this point. In a few weeks things should be more clear.