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.
	
		
			
		
		
	
				
			
	