It could be sudden duckling death syndrome. http://www.backyardchickens.com/t/768976/sudden-duckling-deaths Read this.
- Poop Inspector
Welcome to BYC! Do they have a warm place at night?
How large is the pen?
- Plum Nearly Farm
Probably something like a mink they can get through a 1" hole so very sorry for your loss. At 4 weeks they need temps around 70*