Place each chicken in a large zip-top plastic bag.
Combine 1 can beer, Italian dressing, and fajita seasoning; pour evenly over chickens. Seal bags, and chill 8 hours, turning occasionally.
Remove chicken from marinade, discarding marinade.
Open remaining 3 cans beer. Place each chicken upright onto a beer can, fitting into cavity. Pull legs forward to form a tripod, allowing chickens to stand upright.
Prepare a hot fire by piling charcoal on one side of grill, leaving other side empty. (For gas grills, only light one side.) Place food grate on grill. Place chickens upright on unlit side of grill. Grill, covered with grill lid, 1 hour and 20 minutes or until golden and a meat thermometer registers 170°. Let stand 10 minutes or until thermometer reaches 180°. Carefully remove cans, and cut chickens into quarters.
Marinated Flank Steak: Omit 3 cans beer. Substitute 3 (2-pound) flank steaks for chicken, and marinate as directed. Remove steak from marinade, discarding marinade. Place steak on lit side of grill. Grill, covered with lid, over medium-high heat (350° to 400°) about 20 minutes or to desired degree of doneness. Let stand 10 minutes before slicing.
Note: For testing purposes only, we used Bolner's Fiesta Brand Extra Fancy Fajita Seasoning, which is available at Wal-Mart and most supermarkets.