1. In a small bowl, combine lemon zest, garlic, salt, pepper, and olive oil. Divide mixture between 2 large resealable plastic bags. Place 2 hens in each bag and seal. Turn to coat hens with marinade. Let marinate, refrigerated, at least 1 hour or up to 4 hours.

2. Preheat a grill pan or a grill over high heat. Remove hens from marinade, wiping off any excess zest or garlic. Cut along each side of the backbone to remove it. Place hen, breast side up, on a cutting board. Flatten with the palm of your hand to break the breastbone and collapse the hen. Make a small incision between each leg and bottom end of the breastbone and pass the ends of the legs through the incisions to hold them in place. Repeat with remaining hens.

3. Place hens, skin side down, on grill and immediately reduce heat to low. Place a brick or heavy cast iron pan on top of each hen and grill, rotating frequently, about 15 minutes. Turn hens over and grill until cooked through and the internal temperature reaches 160 degrees on an instant-read thermometer, 10 to 15 minutes. Serve immediately with lemon and olive relish.