The best option if the girls are to stay in the run most of the time, is to buy a freestanding coop, and erect a strong, foxproof walk-in run for them. That way, you can get a coop which is the right size for them to roost in, and provide a run which is large enough for them to be happy in and for you to get inside to clean and handle the birds. For 4 hens you need to plan for a minimum of 8 sq. metres of floor space, and 10-12 will be better if possible. The sides need to be constructed of weld mesh, and the smaller gauges will be good at excluding vermin such as mice and rats, as well as foxes. It's best if the run has a mesh roof, and best of all if it can have a solid roof to keep the floor dry, either Onduline or clear corrugated plastic. The base needs either to be fixed to a sunken fox proof skirt of underground mesh, or else you can line the floor with mesh and fix it to the sides to make a secure box. Minimum height is 2 metres, not only for your own comfort but to help exclude foxes. Foxes can climb over a 2 metre wall so thats why the roof needs to be either mesh or solid.
If you buy a freestanding coop the options are wider - you may like to consider plastic coops, which are easier to keep clean and less likely to get redmite infestations, besides never needing treatment with creosote to preserve the wood and deter mite. I have a Green Frog and am very happy with it, but there are now quite a few good options on the market. If you buy a wooden one, you may like to ask advice on here first, particularly about coop design and ventilation provision, which is crucial. Also the very cheap Chinese imports tend to last for only one season, if that, before leaking of falling apart.
As for the floor, you could consider lining the earth base with permeable weed proof membrane, well pegged down at the edges, as this will prevent them from scratching earth up into whatever bedding you use on top of it. Some people pave the run, but this is colder and harder on the birds feet and doesnt drain so well as membrane, also its heavier and more expensive to instal. You do need to think about how you will dispose of dirty bedding, and personally I wouldn't recommend sand as it gets very dirty and is heavy and difficult to remove and doesn't compost well. I use Aubiose, which is shredded hemp stems, over the membrane, and they can scratch around in this and its easy to poo pick and it composts well. Other people use shredded wood chips, obtained from a local tree surgeon, though you need to be careful what kind of wood is used - thorny bits are not good for the girls feet. I think if you put wood chips over sand on top of earth, you will have a right old mess in a few weeks, especially after rain in an uncovered run.
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