Roof vents can be dome-shaped, square (like the one shown here.), rectangular or chimney-shaped. They all have a cap over the exhaust opening, but not all come with a grid to protect the opening ...

Shift the hole so the top of the vent cap rests on the high point of the lap siding. Pull the vent cap and duct apart to make measuring easier. Bore a 4-1/4 inch hole with a hole saw. Pro tip ...