Nesting box divide ...

The birds will all have a prefered location to lay their eggs and this is usually in the darkest corner. I have ten hens and all lay in one of four nest boxes. Ideally a nest box should be approximately 30cm squared to make it comfortable for them to lay an egg so depending on the size I wouldn't take out the divide. Also, the hens should be forced/encouraged to sleep on a perch. This prevents the nest box from getting dirty and can prevent the birds from getting some illnesses and diseases.

Hi.
I have been trying to get some of my girls not to sleep in the nest box section for months now, because of the mess they make in it which makes for dirty eggs,
I too have been closing off the nest box section at night, but every oppertunity they get, they still go in the nest box.
The conclusion I made is that the perch area is just not big enough for 4 birds, although the info on the coup says 4 to 6 birds. I would never house 6 birds in it.

As a rough guide each bird requires 20cm of perching space. You will need to be persistent with young birds so they no they should sleep on the perches. Some of the heavy breeds however, may struggle with too high perches - my Faverolles doesn't bother with them and since its only her I must confess I leave her be 
A lot of houses state how many you can keep in it and sometimes they can be a bit err cosy. Often though they will say 4 medium laying hens or 6 bantams for example. At the end of the day only you know what you are comfortable with and whats best for your birds.

* blocking!