Rinse the turkey in and out with water, and pat it dry. Set it in the roasting pan, and set aside at room temperature for 20 to 30 minutes before you begin.
If there are giblets inside the turkey, remove them and set aside – you can use them in the gravy.
When you're ready, preheat the oven to 350 °F.
Zest all four lemons. In a small bowl, combine the butter, finely grated lemon zest, cracked peppercorns, and 1 tablespoon of salt.
Take about a quarter of the lemon pepper mix, and rub it inside the bird.
Rub the remainder all over the turkey skin. Where you can, loosen the skin, and rub the butter mixture inside.
Cut two of the lemons in half, and place inside the turkey, along with the garlic, shallots and thyme.
Place the turkey in the oven, and roast for 2 1/2 to 3 hours, basting frequently with the juices that arise, as well as the lemon butter as it cooks. You can add the juice of the remaining two lemons to the basting liquid for an extra zesty taste.
Transfer the turkey to a platter when it's done, and let it rest for about 20 minutes before carving.