How many racks of lamb can be made into a crown roast?
I don't think the amount will matter too much for cooking, just may take a bit longer, but a thermotor should help there.  

What I would consider doing is make the crown out of the three like you had planned and cook the fourth seperatly (but at the same time).  When the lamb is done carve the extra rack (by the single bone) and use it to garnish the crown either inside the crown poking out, or infront of the crown laying on their sides so everyone can see how perfect the lamb is cooked... so everyone will be impressed with both the pretty crown and how well the meat is perpared.