Don't use a hole saw. Use a 4 inch die. You can buy just the die at Home Depot for sheet metal and use a bolt and nut or by the kits for all diameter holes with the hydraulic pump. For sheet metal you don't need the pump style. We cut holes in 1/4 inch thick steel with the pump. The hole is 4 inch not 4 1/8. Well I guess if you use a 4 inch hole saw it would end up being 4 1/8 to 4 1/4. Just did another demon hood last month. The water dams call for a 4 inch hole.
Mine was for a race car application where the grommets weren't needed.