Confit Duck Legs

Mince and mash 4 garlic cloves to a paste with a pinch of kosher salt. Stir together paste, kosher salt (1/4 cup), thyme, quatre épices, shallots, and bay leaves in a large bowl. Add duck legs and toss to coat, then marinate, covered and chilled, at least 1 day and up to 2 days.
Wipe off marinade with paper towels.
Trim off 1/4 inch from top of garlic head, then stick 2 whole cloves into head. Melt duck fat in a wide large heavy pot over low heat, then cook garlic head and duck legs, uncovered, over low heat until fat registers approximately 190°F, about 1 hour. Continue to cook duck, maintaining a temperature of 190 to 210°F, until a wooden pick slides easily into thighs, 2 to 3 hours more.
Transfer duck with a slotted spoon to a large bowl (reserve garlic for another use if desired). Slowly pour duck fat through a fine-mesh sieve into a large crock or deep bowl, leaving any cloudy liquid or meat juices in bottom of pot, then pour strained fat over duck legs to cover by 1 inch. (If necessary, shorten drumstick bones 1 to 2 inches using a large heavy knife to fit legs more tightly in bowl.) Cool to room temperature, about 2 hours, then chill, covered, at least 8 hours.
Just before serving, remove duck from fat (reserve fat for another use, such as frying), scraping off most of fat, then cook, skin side down, in a large heavy nonstick skillet over low heat, covered, until skin is crisp and duck is heated through, 15 to 20 minutes