Introducing a new duckling

Hello i have one duckling which i have hand reared and want to put into the pen with the other 3 ducks i have. The ducks wont let the baby (4 weeks old) into the pen they just keep really pecking him and throwing him out, i lock the pen at night so wanted to know if it is safe to just put him in or can they actually hurt him? sorry if that sounds silly but they seem so much bigger than him and he is frightenend of them

Many thanks!