what is a sure way to tell sex of chicks.I can tell when they are a little older but need to separate soon to move the roosters to a friends farm for food when ready.
There is no sure way that works for all of them each and every time. I've had some 4 months old that I just wasn't sure about. It's usually easier for me to say "That one is definitely a boy" than being really sure one is a girl. I almost gave someone a four month old cockerel when they wanted pullets. That would have been embarrassing.
But most of the time I can tell at five weeks one way or the other. This article gives some things to look for. At that age the things I generally look for are the size and color of the comb and wattle, if any, how heavy the legs are, and general posture and conformation of the body.
If you post photos use one to give a good look at the head so we can see the comb and wattles. A profile post showing legs and posture can also be really helpful. Sometimes there is no doubt, sometimes it is a bit of a guess.
4 weeks is fairly early to tell gender, unless there are some early cockerels showing their stuff. Post pics and we can help decide if you have either at this point. Otherwise, I'd wait until their 6-8 weeks.