Remove leaves from rosemary sprigs, and measure 3/4 cup leaves. Soak rosemary stems in water; set aside.
Process rosemary leaves, walnuts, mustard, and garlic in a food processor until smooth, stopping to scrape down sides. With processor running, pour 1/4 cup oil through food chute in a slow stream; process until smooth. Set pesto aside.
Place tenderloins between 2 sheets of heavy-duty plastic wrap, and flatten to 1/2- to 3/4-inch thickness, using a meat mallet or rolling pin. Spread pesto evenly over top of 2 tenderloins; place remaining tenderloins on top, and tie tenderloin pairs together with kitchen string. Rub remaining oil over pork; sprinkle with salt and pepper.
Prepare a hot fire by piling charcoal on 1 side of grill, leaving other side empty. (For gas grills, light only 1 side.) Remove rosemary stems from water; place on hot coals. Place food rack on grill. Place pork over unlit side.
Grill, covered with grill lid, 1 hour and 30 minutes or until a meat thermometer inserted into thickest portion registers 160°. Let stand 10 minutes. Garnish, if desired.