A typical 2 valve engine has just 1/3 combustion chamber head area covered by the valves, but a 4 valve head increases that to more than 50% area. Because of this flow area of intake & exhaust port / passages leading to valves also get increased.
This facilitates induction of more quantity of charge (air-fuel mixture) and also evacuation of all burnt gases inside the combustion chamber ensure faster, cleaner & more efficient combustion. This engine has 2 intake & 2 exhaust valves compared to 1 Intake & 1 exhaust Valve of a 2 valve engine. These valves are smaller & light weight compared to that of 2 valve engine. 4 Valves (2 Intake & 2 exhaust valves) are introduced to improve breathing means - intake & exhaust process of engine..