They are all very pretty, nice flock. At 4 weeks I'd dif keep your duckling in close proximity to everyone but not with them he is too young yet and the others could really work him over if they wanted too. At least he should be adult size before actually being with them unsupervised. and have a way of escape if someone started to harass him. I am very leery of putting young ones in with chickens especially after reading some of the horror stories on here of how chickens actually peaked young duckling your ducklings age to death or seriously injured them.