Thanks for the feedback on coup size. I emailed a retailer in the UK who was really great, and reassured me the coop will be fine for 4 free ranging birds (you can always trust someone who can't sell you anything). The ladies are really happy in their coop, and one day I'll build them a nice one (with my wifes input on colour etc no doubt), but till now, they are laying, not pecking each other to bits and scratching anything they can get to.
Cheers