Is it possible you're waking them up?
With four people in the same bed, there have to be a lot of different sleep rhythms going on-people moving, rolling over, etc.
You could be waking her up and not even realizing it. I have one kid who sleeps like the dead and one kid who wakes up if you even poke your head through her door to look at her.
We had to put them in separate bedrooms because the light sleeper was disturbed and exhausted by the heavy sleeper going through normal sleep patterns.
The light sleeper needs a really quiet, really dark room with a non-varying white noise going to get a good night's sleep. The heavy sleeper we could stick on a hammock in the middle of a maelstrom and she'd be fine.
I would think that with everyone in actual contact with one another in the same bed the problems would be exacerbated.
