You have 4 hens? You could get away with a relatively small coop incorporated into the tractor design, just a secure box for them to go into at night. Sheds can be built in many different ways, for example I have a large shed which is simply setting on blocks, this is how I am building my chicken coop as well. That would be one example of a non permanent structure. Your 4 hens need such a small coop it would be simple to make it manageable to move around.