Engines generally used to have two valves per cylinder. A Chevy small-block has two valves per cylinder. A Harley-Davidson V-twin also has two valves per cylinder, except for the most recent Milwaukee-Eight, which has four valves per cylinder.
Here is a cylinder head from a BSA 250. Notice that the spark plug is not at the center of the combustion chamber. It's off to one side for a very good reason. By fitting two valves of adequate size, there's no room left in the center for a spark plug.
Quite often in the old days, in an effort to make up for the fact that the flame had to travel a great distance from the offset spark plug to the far cylinder wall, a second spark plug would be installed. “Dual-plug head” was the phrase.
With four valves per cylinder, however, there is space in the middle for a spark plug. Further, two intakes from a four-valve head have the same area as one intake from a two-valve engine, but these only weigh 48 grams. The single valve weighs 64 grams.
There is another point to be made: When the smaller intake valve begins to lift, the amount of flowing area that it produces is the perimeter—the distance around the valve—times the amount the valve has been lifted.
The perimeter of two intake valves of equal area is much greater than that of a single intake valve. The fact that a four-valve combustion chamber exposes valve area much faster as its valves lift means its valves do not need to be open as long.
This gives a four-valve engine the possibility of having a very broad powerband. Ducati’s new Panigale V4, for example, has very little valve overlap; it is practically a tractor engine. And yet it makes 214 hp.