Agree. Higher than the nest boxes. Chickens will normally sleep on the highest thing they can get to. You don't want them to sleep in your nest boxes since they poop a bunch when sleeping. Nobody wants poopy eggs.
It is possible that chickens can hurt themselves getting down off the roosts. The larger the chicken, the more likely it is. This does not mean it happpens often. It is something that happens very seldom, but it can happen and needs to be taken into account. Chickens spread their wings to slow themselves down when they come off the roost, so they can safely come down from pretty high roosts if they have room. The higher the roost, the more clear horizontal room they need to fly down. You also want them to be sleeping below any cross breeze that might be caused by your ventilation openings at the top.
Many, many people have Buff Orps in 4' x 4' coops and never have a problem with this. I just suggest you make them as low as possible and still have them the highest thing available for them to sleep on. In that sized coop and with external nest boxes, I'd suggest about 6" to 9" above the top of the opening to your nest boxes.
Since you are getting chicks, I suggest you block off the nest boxes until they are about 16 weeks old or at least until they are used to roosting on the roosts. Usually my chicks raised by a broody start roosting when they are 4 to 5 weeks old. My brooder raised chicks usually don't start sleeping on the roosts at night until they are 10 to 12 weeks old, instead preferring to huddle in a pile. It's not to keep warm, they just like the company. They may get in the bad habit of sleeping in the nest and not want to switch to the roosts if you leave the nest box open before they learn proper roosting.
Good luck!
